I PMd this to Sorpaw the other day:
GIMP Manual: (refer here when stuck) GNU Image Manipulation Program
GIMP Tutorials: (start here to get the basics) GIMP - Tutorials
Best Cartographers Guild Gimp tutorial ever!!! (work through this whole thread all the way - the most crucial bit is posted half-way through the thread):
[Award Winner] Explanation of Layer Masks in GIMP(and PS)
Cartographers Guild Gimp tutorial (advanced!): RobA's Artistic Regional Map in Gimp Tutorial - Revised Presentation (original thread here: [Award Winner] Using GIMP to Create an Artistic Regional RPG Map)
New to GIMP
Save and practice. Check out the tutorials. It is worthwhile to do exercises to familiarize yourself with terminology. As far as crib sheets go, TAKE NOTES as you go! I have not seen a good crib note site for GIMP 2.8, but start your own at least so that you can replicate your steps! Good luck - - I am doing much the same thing!
I know this kind of goes without Saying but.
Learn your hotkeys! Try to use Gimp with only your keyboard to select tools and options. Saves so much time