How do you disconnect the rear wheel on a 1990 CBR 600

If you find temp sensor for the fan disconnect the wires and connect them together and the fan works then you need to replace the temp sensor, if the fan doesn't work then connect it to a battery and if it works its down to the wiring or a fuse.

Standard settings are...Rear Preload: Position 3
Rear Compression: 1 turn out from maximum (punch aligned)
Rear Rebound: 1 turn out from maximum (punch aligned)
This is for the 97 & 98 year models.

remove both rear side panals about half way along the seat on both sides there is a bolt remove both of them then pull seat to rear of bike.

yes many will.

Torque wrench is not needed, just tightne the bolts securely and fold over the locking tabs(if equipped) if it does not have locking tabs a little bit of BLUE thread locker is a good idea. Be sure to use blue( removeable) and not red ( almost not removeable without heat) so you can get the sprocket off next time.

if you look closely on the side wall of each tyre, all that information is written on there.

remove the rear wheel & remove the rear sprocket and sprocket hub from rear wheel. (should just pull out) inspect the cush drive rubbers, chanches are they are destroyed & dissinagrated....replace them. check your wheel bearings in hub & sprocket plate.check axle. sprocket teeth & chain...reassemble and go riding..

could be the transmission output shaft bearing.
or a dry / worn out chain
or if the rear wheel is not in alignment binding the chain
or the cush drive could be worn out esp. with your wording "takeoff" lol

put bike on motorcycle lift and secure. take off exhaust silencer on right side.remove rear caliper from swingarm. jack bike carefully,loosen axle nut and pullout axle.remove chain from rear sprocket.drop wheel down & back.