How do I know this is easy enough for beginners? Because I just started doing this. Everything that I tried were experiments that I cataloged as I went along. I used Gimp which you can download for free in this link. These pictures are obviously flawed and are not meant for professional use, just trying something new for the first time.
Step 1: Drawing
I used an old pencil drawing of a few characters I created. To get the drawing into Gimp I highly suggest using a scanner. That is the way I would've gone if I hadn't broken my scanner a few weeks ago. So instead I ended up using my phone to take the photo then emailing it to myself. It was a very low quality picture of the drawing which is okay as long as the camera isn't tilted terribly and you can still make out all of the lines.
Windows > Toolbox (I prefer Single Window Mode)
Windows > Dockable Dialogs > Layers & Tool Options
Start Gimp > File > Open as Layers > Select the picture of your drawing
Right Click the Layer > Scale Layer
Image > Fit Canvas to Layers
Step 2: Outline
Create a new layer that I label "Ink" (make sure the setting is transparent) and put it on top of the "Drawing" layer. Trace over the pencil lines with the ink tool (I used size 1). Hold Shift to make straight lines. On a side note, only one of my characters have both eyes showing, instead of tracing two eyes I copied and pasted the first then flipped it (the only problem I ran into with that was changing the reflection circles in the eyes). After you finish tracing turn of the visibility of the "Drawing" layer by clicking the eye symbol next to the layer name in the Layers Tab. Create a new layer that is completely blank and put it under the "Ink" layer. The purpose of this layer is to help get just the right colors in certain situations because a transparent color plus the under color of white will give the specific look you might need (if you intend on using all opaque colors don't worry about this layer).
Step 3: Base Coloring
For each different base color I make a new layer (for a character: skin, eyes, clothes, hair). To color you need to use the fuzzy tool. Make sure "Sample Merged" is checked. Click somewhere in the area you want to color and hold while moving the cursor down to increase the threshold. I pull down and stop right before the threshold spreads farther past the area you want to fill. Then use the bucket tool to fill. Select > None. If anything unwanted (such as eyes in the skin layer) is colored simply go in with the eraser tool.
Step 4: Shadows
Use the color picker to select the base color then subtract 15 - 25 from the value to get the color of the shadow. I used good judgement on where to put shadows. Just remember most objects will of one of two or both types of shadows. The shadow on the object and the shadow the object casts.
Step 5: Transparency
At this point the characters are done and it's time prepare for a background. I used the fuzzy tool to select all the white space around the characters and deleted it.
Step 6: Background/Special Effects
Add a new image layer "Background," the same way you did in the first step. Then time for special effects. This is the step I played with the most. Try different lighting using dodge/burn tool, add gradients, and filters to get the look you finally want.