How do you want your information to appear? 720x486 D1 (pixel aspect 0.9) with letterbox will be the most straightforward way to do it, but you'll retain more information if you use D1 Anamorphic Widescreen (pixel aspect 1.6, I think).
I have had good luck downconverting HD in AE (in order to prep a DVD production file) by dropping my HD .mov file into a D1 Widescreen comp, and then scaling it to... I think it was 45%. (Sorry, it's been a while since I've done this). A little bit hung off the edges but it seemed to scale better than trying to fit everything into that frame at a hyper-specific decimal amount. Then, I was encountering some aliasing issues, so I added a Gaussian Blur with the Blur Amount set to 2, and the whole thing came out quite clean-looking.
If you require a really clean downconvert, though, I suggest doing it in hardware rather than software.
Media Director, Cicada Design Inc.
and Freelance Know-it-all
comp frame size is easy, d1 is 720x486, so that's your frame size.
frame rate should match the footage. use 23.976fps (note that ae should be set to use the more accurate 23.976 frame rate, make sure that ae sees your footage as 23.976 by selecting it in the project window and choosing file>interpret footage>main..., if it says 23.98 set it to 23.976).
then as steve points out, decide how you want to preset the widescreen footage, if you want to letterbox or crop your footage choose .91 par, if you want to maintain full frame widescreen, then set it to 1.21.
when you add the footage, if you choose .91 par, you can choose layer>transform>fit to comp width to letterbox it or fit to comp height to crop it. if you chose 1.21 par, then either should work.
if you need to render out as 29.97 interlaced, then you'll set that in the render settings. set the frame rate to 29.97, select field render (usually upper for ntsc sd) and then choose a pulldown cadence (any should be fine).
Thanks for the advice. I should have mentioned that I was doing a center crop (my bad).
Yesterday I imported my sequence into a 720x486 29.97i comp, resized it and rendered it out with the separate fields (lower) and the first cadence. The footage looked fine in FCP, but I'd like to try it with your suggestion of adding the pulldown on the export, to see if it looks any better. (I'm thinking it should be fine either way, but I want to double check.)