Slacken off the adjsutment cables next to the handbrake in the car. Jack up the rear of the car so both wheels are of ground.
Remove one wheel nut. At about 2.oclock if you shine a tourch you will see a adjusting wheel. Turn the adjusting wheel with a flat screw driver untill the wheel just starts to bind. Do the other wheel the same.
Adjust the hand brake cable adjusters in the car so they just come into contact with the lever in the car. Pull the lever up and hand brake should be applied. It will be handy to have a 10mm deep socket as it makes it easier to adjust in car. Road test to make sure hand brake not binding.
All e30`s with rear disc brakes adjust the same. If not sure read, a manual in one of the car shops.