1. The first step is to create the windows that you want to capture and leave them up on the screen.
2. Just to the right of your keyboard, you should see three groups of keys. The lower set of keys usually is a group of arrow keys. Above that is usually a set of some six keys that are labeled "Insert," "Home," "Page Up," etc. Above that should be three keys that have rather odd labels - like Pause/Break and ScrLk. One of those keys should be labeled PrtScrn/SysRq. Pressing Alt-PrintScreen (Alt-PrtScrn) places an image of the frontmost window on the clipboard. Pressing PrintScreen by itself places an image of the entire desktop on the clipboard.
3. If you press PrtScrn/SysRq, nothing will appear to have happened. However, your computer just took a snapshot of its screen and stored that picture on its clipboard, much as it stores information that you cut and paste.