When I need to show somebody via email how to do something on the computer, and I need to do it quick, I use Paint rather than GIMP.
If you have a PC with Windows 7, you should have Paint available under the Accessories menu. (All Programs -> Accessories -> Paint).
For each important step or set of steps on a given window, you will want to do the following:
1) Make a screen capture of the window with any options or drop-downs showing. Use the “Print Screen” button on the upper right side of your keyboard. (If your “Print Screen” button isn’t on your keyboard, you’re on your own). Print Screen copies the current screen into the clipboard as an image.
2) Open Paint and press CTRL-V or right click-paste. The screen capture should appear.
3) On the View Tab, click Zoom Out.
4) Click the Home Tab again. Now you should see enough of the window to crop it effectively.
5) Click the Select button. Drag the box to cover the part of the window you want to show, excluding the things you don’t want (like your Windows toolbar and other extraneous stuff). If you don’t get it right the first time, just click anywhere on the image and try Select again.
6) Click Crop. If you don’t like how the image looks, you can Undo by pressing Ctrl-Z or clicking the left-hand arrow at the upper-left part of the screen.
7) Use Paint’s shapes and colors to circle important steps, show arrows, etc. Simply select a color and then a shape, and experiment. Remember you can Undo your experiments.
7) Click on the arrow under Select and click “All”; or, right click on the image and click on Select All.
8) Press Ctrl-C or right click -> copy
9) Go to your email and press Ctrl-V or right-click -> paste to insert the picture into your email.
10) if your email doesn’t allow pictures in the body of the email, you may save the image in Paint and attach the file.