Fairly big file, sorry! D:
I'm not great at this kind of thing, but I wanted to try anyway. It's kinda basic, but I think it's a decent method for something quick and fun.
Some things I wanted to point out...
The pencil, watercolor, marker, and sometimes airbrush are the tools I use the most in SAI. They're just the ones I like best. The pencil tool at full opacity is great for filling in flat color. Watercolor and airbrush are good for blending and smoothing, as well as filling in areas where you want soft color and shade variation. And great for quick yet interesting backgrounds.
In terms of composition, when cropping it's a good idea to crop more than just one side. For example just cropping at the feet tends to be a bit awkward, adding at least one more cropped edge to that usually makes for a better picture overall. Also you want to avoid cropping at joints.
And this isn't an ideal example of color. I was a little spontaneous with my palette choices. If you want help with color theory I'd suggest looking up a tutorial on that.
I hope someone finds this at least a little helpful!