http://developer.apple.com/internet/serverside/modssl.html
It assumes that you're going to be generating your own self-signed cert, so the procedure will need to be modified if you're going to use a "real" certificate.
Also, note the rather nasty "gotcha" that can occur if you have Apache set to start automatically (Apache will ask you to enter the passphrase for the server key, but since the system is in GUI mode, you'll never see the prompt. The whole boot sequence will hang.