I think that you haven’t received any responses to this because it’s such a basic After Effects question. (This sort of compositing is standard After Effects stuff.) You might get a better response if you post questions like this in the After Effects Basics forum.
I also recommend that you start by learning the basics of After Effects, starting here.
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Todd Kopriva, Adobe Systems Incorporated
putting the ‘T’ back in ‘RTFM’ : After Effects Help on the Web
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