Here's an explanation of my process for acrylic paints.
Click the thumbnails for lager images.

1. I make small pencil sketches to figure out how I want the
illustration to look.

2. I scan the sketch I like best, and then I try out different
colors in Photoshop.

3. I print the sketch in the size I want, and use a light box
to transform it to watercolor- or canvas paper.

4. I start painting the background.

5. More background colors. If you make a mistake when you
paint with acrylic paints, it's no problem to paint over it again.

6. I keep adding details until I think it looks finished.

7. Sometimes I adjust the colors a bit in Photoshop after
I scan it.