Well I've been doing mine at work in my spare time... all on my mouse, so quit fretting about the tablet lol... So far I've got about an hour and a half into it... Kind of an off the wall theme... but gotta be creative i guess eh?
Heres a few tips I'd like to share since my playing with paint
To achieve more detail you really need to zoom in (view>zoom>large or custom size)
Turn on the grid and zoom into 800% if you really want control and would like to edit pixel by pixel.
For shading since there is no feathered brushes you really need to pick several shades of slightly off colors of what you want to shade, and work around your area... (very time consuming!)
and I've discovered to make your brushes bigger and smaller hold control and hit the + and - keys on the number pad on the right!
Theres a few tips I've discovered since playing with paint (yuck) lol....
Oh yeah, and save often, and if you think you may not like what you are about to do or it may turn out not like you were thiking, save different versions of the file so you can always go back there is no history!