Screencasting if a wonderful yet easy way to SHARE what you're doing on your computer. Using JING - free downloadable piece of software for PC or Mac, you can record your onscreen action while narrating about what you're doing.
Be sure to watch the video on the Jing front page. Jing also has a great Help Center -

Jing is only the capturing software. To post the video online, you need a WIKI, a blog, IM, email or an account on on which to post your video. Here's the link to -

Screencast-O-Matic - will capture directly from their website without having to install the little Jing app.

If you only need to get a portion of an image or screen, for a Mac use Command + Shift + 4 and use the crosshairs to clip a section.

In the Windows world, it is <Shift+PRT SC> or on some laptops <FN> and then <PRT SC>
Then just paste it into your document.

Now you can prepare great instructions for those tech projects for your students!

Here's a great example of instruction on making brochures in MS Office.