Welcome to the Guild Loony77. I'm afraid that I won't be able to help answer your questions as I have not gone all the way through Rob's tutorial myself. I would recommend reading through some other more artistic/photorealistic tutorials (e.g. Ascension's Atlas Style in GIMP) to get a feel for other techniques, to see what might work for your own purposes. This will also help you become more familiar with GIMP as well.

Cheers,
-Arsheesh