You can do it in 3 ways (as far as I'm aware)
1. Purchase Quicktime Pro and select new movie recording from the file menu
2. Download the freeware application called irecord. You can download irecord from - http://www.freemacware.com/irecord/
3. Use imovie to make a video
The second option would be the best because its the fastest way and also the cheapest (you don't have to purchase quicktime pro)
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