Hello. I'm hopefully gonna help you improve your photo manipulation skills with my very first 'ible. That's my intent anyways. And I'm apologizing in advance; I'm a terrible writer. And my 'jokes' suck. Some of you authors are hilarious.

All you need:
An image
Image editing software with layering capabilities and a familiarity with the software.

This is like art class, with advanced mediums. And, since most image editing apps have similar tools, this isn't necessarily teaching any particular software. Instead, I figured I'd focus on the subtle nuances it takes to go from amateur to semi-pro. I don't do this and have never done this professionally... so I'm only offering 'artistic' advice. There're probably much quicker, professional looking methods than mine.

To show how similar the editing softwares are I'm choosing to do this manipulation in Gimp. I typically use Photoshop and have never used Gimp outside of viewing images. So this was a first time for me applying it. As expected, it follows GUI convention; File, Edit, etc. I didn't have much trouble locating tools, settings and properties in Gimp.

Okay, I don't want this to be overwhelming or difficult in the technical sense, so I'm sticking with very basic tools, options, properties, etc. And to show that it can be done.

-Selection Tools
-Rotation and Scale
-Paintbrush
-Eraser
-Dodge/Burn

That's it. Those are the only tools I used; and the paint bucket a couple times. So don't go thinking you need Photoshop CS to make a decent photo edit. Gimp is free; as are many other similar softwares and web apps.

-Blur
-Distort
-Noise

Of more than a hundred, those are the only filters I used. Very different from the blur and smudge in the toolbox.

-Hue/Saturate
-Brightness/Contrast
-Color Balance/Colorize

Are the only color properties I altered.

And of course a load of layers. If you're gonna be working with many-a-layer...Name them. You'll thank yourself.
So if your editor has at least those above-mentioned, popular tools [and has layering capabilities], you may be able to follow step for step with your own subject.

Well, first things first, I needed a subject. I chose a zombie theme 'cause their design is somewhat chaotic...blood, guts, etc; in no particular order. I mean, how hard is it to add blood splatter? [It's actually a lil tricky, I found out] And since this is for many others, I chose a model who's somewhat famous. Maybe you recognize him. All I did was Google the names, went to the images and chose to filter for the LARGEST images. So go LARGE [highest resolution] if possible. They'll have excellent quality so if you scale it down it'll retain some detail you include. It's important, when using a collage of images, to ensure that you don't have one hi-res subject and a fuzzy, blurred or pixelated subject beside it. Attempt to keep them all sharp as possible when they're going to be on the same plane/dimension as the other. Obviously Google had a huge selection of hi-res images of my subjects to choose from in the search. Another thing to keep in mind- If possible attain an image with a background that sharply contrasts the subject. It's easier to crop 'em that way.

Once you have your editor and subjects, you can move on to the first step.

Step 1: Crop and Color

If I have in mind a layout, the very first thing I prefer to do is Selection and Cropping. Now, in Photoshop, I almost always save my layer selections grouped like: Background, skin, clothes, etc. Saves a bundle of time if you gotta go back. If I don't save a selection then I'll copy the selected region as a new layer. It may take a little extra effort, but it's definitely worth while to save selections.
[you'll notice in my screenshots that I may contradict some of my recommendations, but I typically follow what I'm encouraging]

Since Slick Willy has the most prominent presence in my surprise layout, I'll be focusing on just him for editing. Because this is more of an art lesson, you're welcome to use a subject of your own.

You'll notice the mostly-solid background behind Slick. Rather than selecting him, I simply use the Wand selection tool and adjust the Tolerance accordingly to select the blue background. Sometimes you simply can't get a perfect selection, and you'll need to magnify the spots where the selections have gone awry, then use the Free Hand selection tool to get the holidays. If all else fails, I manually select whatever it is I need using the Free Hand selection tool.

After my selection is closed and I'm satisfied, I Invert the selection so only Slick is selected. Cut him out and past him onto a solid black canvas. If the cut is too hard of an edge, select the black background, Grow the selection from the Select options to 1 or 2 pixels. The hard lines can now be blurred, darkened; or you can Feather and Erase the selection. Depending on the situation, I use any of those 3 techniques. But hard lines and sharp corners really mess up the flow.

At this point I recommend getting different selections that are going to have the same properties; for example, Slicks flesh. To get the dead flesh color, I selected out all the visible flesh, Feathered it to 1 or 2 pixels [depending on the resolution, sometimes a 1 or 2 pixel difference isn't even noticeable; so adjust as needed] and changed Hue, Saturation and Brightness or Lightness. A low saturation was used, with a bluish green hue. I didn't put much thought into the color, so be creative. I also used the Contrast adjustment to darken up the folds in the skin. And I made a copy of the selection into a new layer.

Step 2: Color Options

When it comes to color options [this is including Brightness, Contrast, Hue, Saturation] I wanted a pale blue or green skin. With the flesh tones selected I used the Hue and Saturation properties and adjusted accordingly. A low saturation was used. The options are typically on slider bars which allow you to see the image change colors as you move it. Remember to feather the selection to at least 1 pixel before you color, or you may end up with edges that are too defined. As you lower the saturation you'll retain your new color, but it will be pale and lifeless. If you want your result to be more realistic, use subtle, very subtle colors. My intention was to have an almost CGI game feel.

* Gimp does have layer transparency options, but there're quite a few and I wanted to limit the tools for the sake of simplicity.
*And unless I mention otherwise, I'm usually referring to the topmost layer. In this case, it'd be Slick.

Step 3: Some Prep Work

Since I intend on shredding this zombie, I want something under the flesh.
The skull I chose had a similar angle that Slick's has. If the skull's teeth were to be exposed and they didn't line up with Slick's, it'd look off.
Doesn't have to be a perfect match since I don't plan to have much skull visible. The point is having something under Slick's layer instead of just empty canvas.

I Flipped the skull and dropped the layer under Slick.

Step 4: Some Detail

I figured I'd leave the hole I just made in Slick, and touch it up.
I first made sure I was on Slick's layer, Selected the hole [the transparent space I cut], went to the selection options and turned the selection into a Border. Depending upon the resolution, the Border selection width will vary. Since the flesh on the cheek is thin, I made a somewhat thin Border selection. And I of course Feathered the selection to at least 1 pixel.
To give the wound some depth, I used the Emboss Filter on the selection. This is another tool which allows you to preview the results before committing them; so adjust accordingly. If you're not really artistic, I recommend playing with options until it 'looks right'. Even if you don't have an artistic eye, you'll still be able to tell if something doesn't look quite right. And being able to preview changes in levels, colors and thresholds as you alter them is valuable, less time consuming too.

Step 5: More Detail

You can't simply make a hole. I mean, where did the flesh go? We need flaps of skin hanging.

Notice the Free-Hand Selection I made in the 1st image. I'm not adding flesh, I'm just adding the shadows of the flesh. I made the selection so that when I Burned it, it wouldn't Burn the skin flap, but faded it out in the other direction. This is when a slightly hard edge is acceptable. The skin flap is away from the face, the shadow then fades out. It was all done with the Burn tool; less than a minute.

The inside of the cheek certainly ain't flesh-tone. So what I did was make a another Free-Hand Selection representing the inside of the cheek. Just a lil' piece of detail is all you need sometimes. With that piece selected, I went to the Colorize option and changed the Hue to red. Deselect and add add a Burn to the inside cheek to give some depth. Don't be afraid to go dark with lots of contrast. I sometimes keep high-contrast, black and white copies on the bottom layers; so I may select shadows easily.

Puncture wounds [holes] are going to have almost black pits, fading out as you come to the surface. To angle the hole, simply move the cavity in the appropriate direction. Such as the cavity abruptly stopping at the cheek bone but fading towards the mouth and gradually out on torn skin.

Burning the eye sockets will also give depth. Try to keep some highlights around protruding areas like the lids and brow line.

I'm pretty certain most editors are going to have an undo, so, nothing to fear.

Step 6: Massive Headwound Willy

I'm thinking it's gonna take more than a puncture in the cheek to keep a zombie down. A bat upside the noodle might detour him.

Using the Eraser or Selection/Feather/Delete, I removed part of Slick's forehead. Unlike the previous cavity, this one contains 'something'. Burning the perimeter of the hole will add depth. I then moved to the Skull layer and Burned around the edges of the opening, hid some of the flaws and Sharpened it a tad.

When it looked suitable, I used the same Embossing technique as on the cheek. Select and Feather the around the edges, gauge the Emboss levels in the preview. All this really does is give a ragged, hard look to define the edge. While I had it selected I Contrasted it a tiny bit and Colorized it to a red hue.

I decided this bat made it through his skull, so I Googled a brain image and dropped it behind SLick's layer.

Step 7: Shading

I'm a high contrast artist; I've been told.

Don't be afraid to go dark. Don't be afraid to go too light either. When I was a kid I had a difficult time shading 'cause I felt as if I was blacking out my hard work on the detail. Once you can work with both detail and shading, things'll start coming together.

Keeping with the same lighting as the image, I simply Selected everything except the elevated, torn skin on the shadow side of the head. There should be almost an abrupt end when the shadow gets to the overturned protrusions. Again, the selection is feathered to at least 1 pixel.

After the selection was feathered I lowered the Brightness, increased the Contrast, some slight Noise was added. I don't have any particular order in which I do these multiple steps. Sometimes I go through numerous Undos, swapping between different techniques until it looks good enough. Only advice I can offer at that point is patience. It's supposed to be for fun; armed forces aren't coming for you if it don't look right. There's no 'correct' way to create a nice piece of work, only more efficient ones.

With a Feathered Selection and Colorize, I Freehand Selectioned a blood stain on the collar.

Stereotypical zombies have concaved features, so I removed a bit of the left cheek or jowl with Burn.

Finally, some splatter on the collar and blood weeping from the eye sockets. Did I forget to mention I removed his eyes? In case I did, select the whites, feather and clear. A hint of red or pink on the lower lid will give the depth.
The blood trails were created with a Free-Hand Selection, keeping them tight, thin and flowing with the contour of the face. Feather to 1 and Colorize a dark, dark red. Colorize retains the shaded features. The splatter can be made with varying red hues and a sponge or similar Paint Brush. Include both lighter and darker reds; use it to cover flaws; whatever. Be creative.

Step 8: Release the Attack Dogs

If the bat doesn't work, perhaps a German Shepard or Pitbull might slow it.

Whatever method you like, remove a chunk of chin on your subject's layer. I used the Free-Hand Selection tool and made a random, Feathered cut. Try to make it jagged and torn, if you like. Once the selection was Cleared I Embossed the edges, same manner as in the two previous injuries it sustained.

I Freehanded another Selection for blood on the chin, Colorized it a red hue and reduced the Brightness.
Still didn't look quite right, so I went to the Skull Layer, Selected just the visible chin and Embossed it.

Step 9: Elite Forces

Frags are for newbs! This zombie's next opponent is equipped with a hatchet.

Took some looking, but I settled on a Google image of this CGI hatchet [it ain't real] that had the perfect angle. Cropped and Resized it. The original was much lighter. It's very important to keep all the clipart you use at the same contrast if you want more believability. So while it was selected [and feathered], I adjusted the Brightness and Contrast 'til it looked good [acceptable for the time being].

Next I needed to get the axe inside the head. The second image shows just the axe and the new layer I made from a selection of the top layer. This is so I may bring the new selection of the brow in front of the axe. Add some Burn between the Brow and Bridge of the nose to 'fold' it.
Finally, the axe head that's buried in the skull needs to be shaded. Move to the Axe layer and Burn dark, almost black around the flesh; Fade the Burn as you move toward the handle. *You're not actually fading the Burn, just not applying too much.

I'm going to assume most editors that include a dodge/burn tool also give you the ability to adjust the rate / exposure. Gimp and PS have this ability.

Step 10: Fill in Some Holes

That gaping hole's been bothering me. Took no time to Google again, find a crowbar with a nice resolution and perfect angle. The background was mostly solid and it cut out easily. I pasted the Crowbar as a new layer.

In the first image, you'll see the cut I made across the nose and face to create a new layer to go above the crowbar. So on Slick's layer, I Selected the region that will above the Crowbar layer, Cut it and pasted it as a new layer. Better to select too much, you can always erase, fade, blur or sharpen any overhang. Where the crowbar enters or exits should be dark, this can also cover some overhang you may have on the layer.

I used the Magic Wand Selection tool to select a random section on the Crowbar layer, Feathered and Colorized it red.

Now I'm moving on to finish this with Pixlr.

Step 11: He Got Camped, Hard.

Never used Pixlr. Hadn't even heard of it until reading of it on Instructables. I decided to switch over to Pixlr to finish up; show how similar these editors can be.

Inevitably, some camper ends Slick's kill streak with an Olympia in the back of the head. Let's see the aftermath.

Everything was easy to find on the Pixlr site. Didn't even get a hiccup 'bout browsers or settings.

I think you can see where I'm headed in the first image. Eraser tool. It was that simple. I loaded my image in Pixlr, Duplicated the Layer and started hacking at this guy. There's a technique to hacking though, adjusting the depth, contours, etc. I give an example in the first image, back of the head. I suggest playing around, Erasing your subjects head with different brushes and settings. See if you can get different depths and effects.

The second image is intestines. I dunno...I needed filler. Looks like something that might've prolapsed from his skull. I wanna say too that I'm sometimes only using these background layers as reference, shading guides or as textures. The intestines have a form that I can work off of, with the added benefit of more gross stuff coming out his lid.

Step 12: Finish Him

At this point, you can pretty much go crazy.

The first image is of a section of those intestines I've been saving. The next image is of the intestines filling the skull.

What's happening there is that I'm using the residual curves, folds, shadows, etc. that're in the intestines. Burn them. Burn 'em 'til only lumps of brain are visible. Use the shadows of the intestines as guides. I was actually finding myself playing around with the intestines, experimenting with different effects. Try to make convex or concave areas in the skull. Practice fading the Burn and or using different exposures. See what happens when you change Burn settings to burn highlights or shadows instead of mid-tones.

Once you've obtained the desired shading, it's time to add highlights. [see more shading tips on the images]

Like Burn, Dodge also has exposure settings and the option to Dodge Highlights, Shadows, or Mid-tones.

The light is coming predominantly at his left front face. Some highlights may already be placed correctly and just need a few swipes of the Dodge tool. Other highlights may need to be created from scratch like we did the shading. Just keep it inperspective with the rest of the lighting.

As a final touch, change your Dodge tool setting to dodge the highlights, then hit all the previously highlighted spots once again.

Plenty more touch-ups could be done here, but I think practice is most important. Try not to be tempted to use filters, practice with just a few tools such as Selections and Dodge/Burns. Once you've mastered them choose some new tools and techniques. There're so many shortcuts and options that it just takes time, practice and patience.

I realize there're two challenges involved here: Creativity and Software knowledge. Unfortunately I can't teach creativity, but hopefully I've given you some inspiration to take your photo manipulation skills to the next level.

I really enjoyed creating this, my first 'ible. I've been mooching too long; time to contribute. Hopefully win a cool prize.

Thanks for the interest and I truly hope this helps.