EDIT..........:
!!This post was supposed to be a reply to someone asking about putting 90-91 rear discs on an 89 si....!!!

sorry.

its so easy, its lame. just be confident. it took me about an hour for the full rear swap. just make sure you have all the parts:
-90-91 Rear Disc trailing arms (Civic/CRX Si)
-90-91 E Brake Cables (Del Sol ones might work, not sure)
-Rotors, Pads, Calipers......mandatory stuff
-Proportioning Valve from 90-91 Civic CRX Si or 90-00 Integra (Will work without it, but would be better with it.)
Real easy......:
Trailing Arm Disconnection...Take out the following bolts to pull the trailing arm out:
-Disconnect ebrake cables from ebrake lever (2bolts for bracket, adjusting nut to relieve tension)
-(2) Bolts connecting the main trailing arm bushing to the car (Biggest Round Shaped bushing)
-(1) Bolt for the toe adjustment (At the end of the trailing arm closest to the front of the car)
-(2) Bolts connecting the upper rear control arm to the car (Look to the side of your strut in the back)
-(1) Bolt holding the bottom of the strut to the lower control arm
-(1) bolt connecting the lower control arm to the car
-(2) Bolts holding (2) brackets for the ebrake cable.
Do this for each trailing arm. Installation is reverse of removal. Remember, use jackstands.