In the old days, mattes were created by painting the scene or effect onto a pane of glass. The film would then be shot through the glass, making whatever was painted on it look like part of the scene. The mattes were painted by skilled artists who could match the tones and colours of their imaginary scenery almost perfectly with the surrounding area making for an almost undetectable blend.
These days of course, mattes can be created digitally by anyone who has a computer, an editor and a couple of hours to spare. This video will show you how to do that very technique for yourself. I use Sony Vegas Pro 10 in the tutorial, but the method should be the same for any of the Sony Vegas Pro series. It's not difficult and the results can be very impressive - Why not try it