There are all sorts of writeups on this, both here and on the web. Search around and you'll find them. One thing I'll add from ugly personal experience:

I'm assuming that since you're talking about a 90 model with a 1.6 SOHC that it's OBD0 with a single O2 sensor. If not, the following may not apply. If so, however, do not--repeat DO NOT--attempt to get around the need for two O2 sensors with the OBD0 B16A by joining both O2 leads from the ECU to your existing single O2 sensor. This will seem to work fine at first, but it will land you in a hell of intermittent, unpredictable and very severe drivability problems that will drive you buckwild crazy. Ask me how I know. DO NOT believe anybody who tells you otherwise, because scads of people will. Again, ask me how I know. Just bite the bullet and either get a dual O2 exhaust manifold or have two O2 bungs welded into your Y pipe. Of course, if you're upgrading to ODB1, none of this applies.

The swap is worth all the headaches, though. I've had my B16A CRX on the road and running reliably for about a month now, so I finally have a feel for driving one of these things. It's a riot to drive--not the fastest car by any means, but plenty fast enough to have fun--and the 34 mpg average fuel economy I'm getting is 10 mpg better than the best of the other faster cars I own. That's worth a lot these days.

Troy
1989 CRX Si B16A
Squalor, Austin, Texas

"After a couple of drinks I'll think straight...." -- Tones On Tail