In shining technicolour, woo! Otto was done just with the blend tool, then I figured out the dodge/burn tool for me. I even sort of figured out how to fix the guidelines for me (er,myself), though sadly I forgot to fix the hat placement before doing all the colouring in!
Btw, is there a way of drawing straight lines on Gimp? Photoshop, I mean. I think they’re largely the same.