Step 1: On layer 0, use a light colour, such as light blue, to make your preliminary lineart. It's easiest just to make broad geometric shapes in the same shape as your subject. This makes it easier when you go to "ink" your lineart, and ensures it will be proportionate (that is, if you DREW it proportionate!). Skipping this stage could cause your lineart to look disfigured, unless you've practiced enough to skip it.

Step 2: Next, go to layer 2. Use the pen or pencil tool (or whatever else you prefer) to make your lineart. Its good practice to experiment with different colours of lineart, but black is usually a safe bet. You can now erase the preliminary blue on layer 0. Tips: If your lineart looks too dark or intense, use the erase rectangle tool on low intensity and shave off a layer of it. It will make the lineart look finer and light, and when you go to colour the lineart wont look as separate from the colouring.

Step 3: On any layer lower than your black lineart, fill it in with colour! (but PLEASE dont use the "fill" tool. Not only will not work on a lower layer, but it looks ugly when people use it and it never goes to the edge of your lineart.)

Step 4: This step is just shading and detail. I'd go further into how to shade, but this isn't a shading tutorial! I just have 1 tip: Avoid shading dark areas with straight black. Shadows aren't black, they're merely darker tones of the same colour. However, if it's your "style" to shade with black, by all means go ahead.