There are instructions in the owner's manual RE child seats. Read them. I'll attempt to summarize and add to below (please correct if/where you see mistakes) but the owner's manual shoud be considered official/recommended...
Rear-facing "infant" seats:
- Generally for infants under 1 year of age up to 20-30lbs and 27-36 (weight/height limits vary by model - our baby passed the 21lb/27" limit on our infant seat before she was 7 months so we had to buy another).
- Got airbag, no dice.
- No airbag, see see instructions below about getting belt tight.
Forward-facing "toddler" seats:
- Generally for toddlers over 1 year, 20-40lbs, over 34"
- Often the same physical seat as the next type described, but for this weight range you use the 5-pt belts attached to the toddler seat to secure the kid.
- In the NSX these require an anchor be run between the headrest and backrest of the seat to an anchor point on the firewall/bulkhead (you have to buy anchor point from dealer an install into pre-existing threaded hole). The instructions (in 1991 owner's manual) cover this completely, include how to modify (cut a hole in) the trim covering the firewall...but you can skip the trim modification and remove that trim section temporarily.
- Make sure these instructions are in the owner's manual for years with an airbag to verify the airbag isn't an issue.
- See see instructions below about getting belt tight.
Forward-facing "booster" seats:
- Generally for toddlers/children 40-70-100lbs (depending on manufacturer), over 34"
- Often the same physical seat as the previous type described, but for this weight range you use the vehicle's seat belts to secure the kid.
- Can also be booster-only style - just thing for kid to sit on, no back/headrest/etc like the previous.
- Not sure about use with airbag. Like previous I'd assume it's fine, but best to take the owner's manual's word, not mine.
(I wasn't sure which of the last two you were talking about...so I decided to cover both...which made me want to be thorough so this thread could be a reference for others - but not so thorough as to cover "convertable" seats which are a mix of the first two.)
Seatbelt tightening instructions...
This is for child seats where you're using the vehicle's belts to secure only the seat...i.e. the first two types. For me this works better than that stupid metal locking clip (which I think makes things worse, not better, because of how hard it is to get the clip on AND the seatbelt tight at the same time).
The NSX's seatbelt (like on many other Honda/Acura products) doesn't ratchet once pulled all the way out, but you can still get it to lock securely.
Put passenger seat ALL the way back then put in the baby seat and attach the seat belt properly. Next, pull the seat belt as tight as you can (your hand will be up near the headrest, pulling away from seat toward where belt goes into pillar, and you may be kneeling on the baby seat) then yank in the opposite direction as fast as you can. That will cause the belt reel to lock-up (until there is slack again). If done right only a small amount of belt will have come back out before it locked up...so the belt will be close to as tight as it was before you yanked. You can then use the seat forward/backward switch to move the seat forward to get the belt to desired tension (and keep it tensioned so the belt doesn't unlock).
I got the idea here from this post in this thread:
http://www.nsxprime.com/forums/showthread.php?p=788086