On the Screencast-O-Matic website there is a quick demo video that gives a great overview of how to do a screen cast.
Screencast-O-Matic
Screencast-0-matic is a free website that allows you to record everything that you are doing on your computer along with your voice and/or video. You can use Screencast-o-matic to create tutorials and lessons for your students. You can also have students use it to create tutorials and lessons for each other.