People have different ways in which they change them out, add them, etc and depending on how many iso's you'd like, will obviously change the end result height
Most common I think is to jack up the back end and put two jack stands on either side, obviously remove the tires, then the diff should drop low enough that the springs come out for you to add the new upper isolators to the pre-existing ones (making it have two up top now on each side)
DONE
You can add them to the lower spring perch but it's easier just to put them up top since the spring would already be out of the way
Changing the rear springs is the same process except you have to take the jack and jack up one side (which extends the opposite) and throw your new, taller springs onto the extended side, then do the same to the other and DONE with that also