You don't.
At least as far as I know you don't.
This is one of the reasons why DirecTV and cable companies offer boxes with integrated DVR/Tivo functionality. The ability to record HD must come under a fair amount of copyright protection so things like stand alone Tivo, Replay TV, and DVD recorders can't handle 720p or 1080i inputs, even on component video.
I refuse to buy a DVD recorder because of this lack of ability and am perfectly happy paying 10 bucks a month for DVR service from my local cable company.
If you find something different out, I'll be happy to be wrong.