Mate I will have a try at a write up for you, anyone feel free to add anything I may have missed I have changed mine that many times it comes as second nature now.
Obviously, jack car up and place on safty stands, the higher you can get the rear the easier it is.
If you can get acces to a hoist this is better.
Take both rear wheels off
Take hand brake off, making sure youve chocked the front wheels
you need to take the bolt out that holds the shock to the hub (19mm from memory)
unscrew half way then use your jack and jack up under the hub, this will allow you to take the bolt out easily,being stock springs they will still some what be under tension, and once you release that bolt they will shoot the wheel down if you dont have the jack there.
once bolt it out lower the jack slowly and get rid of it for now.
If you are doing it from jack stands you then need to undo the bolt that connects the sway bar to the IRS frame (once again from memory 10mm and 12mm), this is not under any real load, once out this will allow you to pry the wheel down far enough to get the springout, youy may need to use a tire lever to lever the spring out. you can use a spring compressor but they are fiddly to use and not 100% required.
If you are using a hoist you dont need to undo that connecting bolt you can actually get good veverage and pop the spring out.
Once the spring is out take the rubber bung out of the top of the standard spring put in the new spring, being a lower spring it should easily slide in.
locate spring, then use your jack once agian to jack up the wheel to align the hub hole with the shock(there is a little locating dowel/extension of the shock hole that sits just inside the hub hole, make sure you sit it in right)
tighten bolt up and then lower wheel, do up sway bar connecting bolt you might need to be a bit rough with this as it may not be 100% lned up but wriggle it in and it will go.
put wheel back on
That is one side done/ repeat for the other side and your done!
just a note when putting your wheels back on and lowering the car, make sure that the metal bung on the underside of the car is sitting in the rubber bung in the top of the spring/ if you are runnig really low springs and stock shocker like I am, then the spring will sit up right and requires me to guide the metal bung into the rubber bung whilst lowering it slowly.
And your done
if you want you should then get a wheel alignment (I dont bother with this as my tires dont last long due to my driving any way)
sorry about some of my terminogy as some of the proper names for things escape me at this time, but hopfully you will understand whatr I mean when you stick your head under the car.