I've been drawing with pen on paper since I was four but I could never get the hang of painting. Illustration is, at least for me, very different than painting.
I've been interested in digital painting for a while now. I tried doing it with a mouse but never got anywhere. I then got a little bamboo tablet but had some trouble figuring out how to do more or less photorealistic art. The process I use for illustration just did not work for painting or digital otherwise. I looked at other tutorials and got hints at what I was doing wrong or more accurately how I was thinking wrong. This is all about what I've learned.
This kind of art can be done with a range of software from GIMP to Photoshop. You'll need a program that uses layers and has a smudge tool. The difference between programs is significant but the method here can be worked on any of them.
At a minimum a tablet is important to keep the feel of drawing. I have a small Wacom Bamboo but I was able to get a hold of a old Fujitsu tablet that has a built in Wacom. Being able to draw on the surface that the artwork is on is a huge advantage but I've done this on my Bamboo also.
This is a raw outline of how I've been painting. I haven't settled on any one method yet, you probably won't right away either. Be willing to practice and experiment to find your style and a process that works well for you.
Step 1: Sketching
The first step is to do a rough sketch. This is pretty standard and comfy but it led to a problem. When drawing on paper, the sketch lines guide my process. Early on, when digital painting I had one of two problems, either I was painting over my sketch lines and effectively erasing them or I'd have to erase the lines later.
For this, all you need is the vaguest of outlines. If you're an illustrator like me, you'll want to strengthen details that you like in your sketch. You'll want to clean up lines and refine things. Resist that urge. Anything you do in this sketch is going to be removed later. Don't invest any extra time here. This is part of the philosophy that I had as an illustrator and had to overcome.
The solution to this problem is relatively simple. Layers. There are many ways I've used to do my sketch lines, if you look closely you'll find that I don't quite follow either of the processes I describe. This is because I'm still experimenting and because I forget to take steps that would save me time and effort. Choose one that you're more comfortable with or develop your own.
Leave the background blank for now. I'm not worried about the background for this picture so I won't be working on that. Create a new layer for the sketch lines. This way you can paint a background and not disturb your main artwork. Then, with your sketch lines in place, create a layer on top of your sketch lines. Set the opacity to a value that allows you to see your lines under the art, usually 90% works.
The pitfall of this method is if you start using multiple layers to paint on (which is a good idea) you'll have to set the opacity on all those layers or you'll have some that block out the sketching and one that doesn't. At worst, I've made the mistake of merging my main layer into a layer that's at 100% opacity which locks the paint at it's 90% opacity value.
If that scares you at all, maybe you should go with the next option.
Alternatively you can place the layer that you'll be doing your painting on under your sketch lines. This way they're always on top. In this example, this would work easily because my lines are thin and don't obscure much.
The pitfall here is that you may not be able to see what you're painting with the lines hovering over. This will greatly depend on your style and the subject you're painting. You'll want to sketch with a very fine line if you're going to paint under.