If you want a simple method to get a screenshot, press the print screen button located above "insert" on a standard keyboard, in the top right.
That will save the image to your Clipboard. Now go to any image editing program, (even GIMP lol), and paste it in with Ctrl + V, or Edit -> paste in GIMP.
Shrink it and crop it to remove your browser stuff, and there you go.
Great tutorial btw, I'm getting into GIMP, looking forward to more stuff.