I used GIMP, or rather, GIMP Paint Studio. GIMP can be downloaded for free from here. The GIMP Paint Studio plugin can be downloaded here. The same principles I use can be applied to other programs--the reason I use GIMP is because it's free. :)
Step 1: Sketching It Out
I checked "Opacity" and "Size" under "Pressure" in the Paintbrush settings, and, using the default Round brush, I started sketching out my picture. The sketch doesn't have to be perfect, but I played around for a while until I was happy with it.
Once I finished the sketch, I selected the background layer and added a simple green gradient.