This blog from Ted Curran provides a detailed guide and examples of how to use screenshots to make it easier for people to see what you are talking about. The page includes links to a range of free screen capture software and examples of how they can be used to work effectively for you to get your message across.
"Screen capture software lets you take an image or a video of your computer’s desktop, annotate it with text, arrows, shapes, and more, and then post it online or email to friends. There are many great free and low cost options for screen capture software for Mac and Windows." (Curran, 2012)