just the same as replacing standard suspension,
removing the old
-undo the bolts on the wishbone at the bottom (dont let the hub assembly hit the floor too hard lol)
-undo the top mount, (i found this very very hard, after 15 years of being tight the nuts didnt wanna come off too easy haha, after 2 tins of wd40, half a day tryna weld new nuts on top of the old rounded off ones and attempting to cut the damper rods I finally grinded the old top cups apart and got some new ones) hopefully you shouldnt find this bit so hard)
fitting the new
-line up both the coilovers and the old shocks next to each other on the workbench then set the new ride height (this is the easiest way to determin how much of a drop you have i.e look how far apart the top mounts are from each other if the lower mounts are equal if you get me lol)
-fit the new hardware int he reverse order you took the old kit off, just refer to the coilover handbook/haynes manual for tourque settings
-make sure the tracking isnt totally out of whack (mine was from all the bashing around getting the old shocks off) if it is i know rapid fit (ford) by me do all 4 corners for 25 quid, shouldnt imagine its much more or less than that.
its the same out back, whip off the springs and dampers and fit the coilies in place, there should be a fitting kit with the coilovers unless they are second hand and you can always use a haynes manual but picture it with the coilovers not standar shocks, hope this helps (y)