How do I get a brush entity to display what a camera sees?

[Karmah]

How do I get a brush entity to display what a camera sees?

[B]Edit:[/B]
New question:
How do I change the size of the screen on the brush? It just keeps looping the picture instead of stretching it across.

[benjojo]

Look at the level included in the source SDK where it clearly shows how the valve mapper did it.

[Karmah]

Thanks, I got it all working now