This is for older vehicle. Remove the lower nut first.Let shock expand.Remove upper nut. Replace shock. Put top nut on first. Apply steady pressure to bottom, allign, replace lower nut.
Raise and suitably support the vehicle. Next, remove the upper mounting bolt(s). Then remove the lower bolts. Remove the old shock and replace with a new one.
The best thing to do is to jack up the vehicle to let the wheels hang down low. You can then pull the 2 bolts off the shock and remove it and then you can reinstall it the same way you took it out.
The process varies from car to car. On some cars they just unbolt and the new one bolts on. Other cars require compression and removal of an entire strut assembly.
