Make sure you put axle stands under the chassis rails at the right
locations (will be somewhere in front of subframe and ensure the car
has sat stably down on them and they are not sinking or tilted, never
work on car on a jack. When it's up on stands the suspension extends
and the shock tops out so it won't extend much more (about 1/2 inch,
it's just working against a bit of torsion in the rubber bushes in the
suspenion arms) when you remove the one lower bolt and 2 top nuts.
You can get to the lower bolt from under the rear of the car but it's
easier to see with the wheel off.  The forked end of the shock just
pulls away from the bushing.  Then when you undo the top nuts the
shock falls out on the floor so tie it up.   You will need a pair of
spring compressors as the new shocks won't come with the spring fitted
unless you buy an assembly.  If it's sagging get new springs.  Make
sure you have taken the spring load off the top mount before removing
the nut or bits will fly.

Take wheel off before ordering shock and check rubber gaiter inside
spring, if they have fallen apart into rings (like they do) you need
them as well (ones on RS13 which is same as Z31cost nearly as much as
the KYB shocks).  You are supposed to fit new nuts and bolts as they
are a type of self locking nut.  If it's been used in an area that
uses road salt take a look at the suspension arms for rust, if you see
any "warts" knock them off with a screw driver and then replace the
arm as you will find a hole under the wart (some people weld patches
on).   Also check the rubber bushes at the front of the subframe, they
are silcone filled and can leak goo.

Subframe along with diff, drive shafts, suspension arms, brakes and
hubs stays attached to the car.  Don't undo any nuts or bolts on the
suspension links.  They have to be tightened when it sat on it's
wheels so you need a platform car hoist to do that.  Or  before fiting
the shock you put a jack under the hub so it's center is the at the
same distance from the wheel arch that you measured before taking it
apart. If they are tighened with the suspension dropped when it's on
stands, it will put the bushes in torsion when it's down on it's
wheels and they may suffer from the rubber peeling off the metal.