Basically you jack up the back end on stands and remove all that is not completely flexible. Support brake disc/drum on trolly jack and wood, and then remove the lower shock bolt. This bolt may be a real beeatch to come out.
Lower trolley jack slowly always taking the weight of the axle on it. Never let it hang.It should drop enough to get the old spring out and the new one in.
About an hour each side. Thats allowing for the lower shock bolt being a pain in the ar$e.
Once you take the wheel off, all should become clear, and as it's a GLS I don't think you have any self leveling bits and bobs to worry about.