Replacing the REAR springs is more work in that you have not only the under carriage work, but also you'll need to remove the rear seat and rear shelf to get at the top of the struts. Although this seems like a big job, it's pretty straight-forward and only requires more time to complete: it's not a hard job and is pretty intuitive once you get into it. I'd recommend doing this in two steps: the undercarriage first, then the interior removal second. I took my time and did it ( me only -- no help )over a weekend, but you could likely complete the whole job in one full day. Again, wear nitrile gloves and keep replacing them if they get dirty while handling the interior pieces (seats, interior panels around the rear window and shelf, interior trim). Keep a few plastic Cool Whip containers: one for the undercarriage small parts, and another strictly for the various interior clips, nuts, etc as there are quite few of these for the interior. Some of the interior push pins are different for various components, so keep track of "what goes where" so you can properly reassemble. You don't have to remove anything in the trunk, so use the trunk to safely store your interior panels, DSP panel, etc.
I started with the right rear, but took more pic's of the left rear disassembly. The process is more-or-less the same for either side other than some minor differences due to the fuel evap system on the drivers side behind the splash panel. Most of the pic's are of the drivers side rear, although this particular pic is the passenger side rear showing the gas tank fill pipe once the splash panel was removed.
This one's the right rear where I've removed the plastic wheel splash panel by taking the plastic push pins and screws out near where the bumper end attaches.

Remove the plastic holder for the brake warning /sensor wires( again, plastic expansion/push pins that BMW uses for many trim components).
You'll also need to remove the small "mud-flap" panel (visible under my hand in this photo) at the bottom front edge of the rear wheel opening. It has two screws you access from the bottom and the push pin shown. The screws hold the mud-flap as well as the leading edge of the wheel well splash panel.
The splash panel is is a big awkward panel but lightweight & easy to take out by moving/twisting it around until you have it out. Once it's out, you now have full access to the strut.
Remove the headlight leveling sensor from the lower control arm to avoid breaking the arm. My car is a 2000, but earlier cars may not have this. This sensor is only on the rear right side -- none on the left rear. It's a 10mm wrench & 10mm socket (I use a 1/4" drive & sockets for small stuff).
Remove the large bolt the attaches the bottom of the strut to the rear suspension. You'll need a breaker bar & socket as it's a large bolt. At this point, only the three top strut nuts are holding the strut assy; however, you'll need to switch over to removing the rear seat and related panels to get at those nuts that are under the rear shelf behind the rear window. Now's a good time to take a break and clean up parts, put on clean gloves, etc before you start the next phase of handling interior parts.
Start the interior removal by removing the lower rear seat cushion ( pull the front edge up and it easily slides out). Also remove the headrests by simply pulling them up and out. Remove the plastic trim panel on the door sill by removing the nut at the back end, and then prying the sill panel up...it has "push pins" that snap into holes on the sill, so use a small pry bar or large flat bladed screwdriver to pry/snap out the 3-5 locating pins. Once out, put the panels in the trunk and the attaching pins in a container so they don't get lost (or keep them on the panel itself...they slide into grooves on the underside).
Here's a pic of the lower seat with the cushion removed so you can see where the trim panel bolt is (just behind the seat belt receptacle...you'll see it along the inside of the door sill. Remove the nut holding the inner seat belt (you don't need to remove the center armrest section ). Next remove the nut for the seat backrest, and then lift the backrest up and off to the side out of the way ( seat belt is still attached ). The seat backs are separate pieces for right & left sides.
'Looks scary with all the rear interior removed, but just move the seat pieces around so you have good working access.
Your car probably looks something like this right about now, but you're almost done!

Remove the plastic covers from the rear shelf attachment points on the rear shelf and remove the bolts and spacers. Remember which-bolts-go-where and how the spacers go so you can reassemble them properly! Great use of a digital camera to keep track of parts and "pre-dissasembly" pics!
Remove the C-pillar interior panel by removing the light and then removing the 3 bolts behind the light assy that hold the panel to the pillar....the panel then lifts and slides out...leave the light attached if you want.
Pry off the speaker grilles, remove the little torx head screws ( 2.5mm??) If you don't already have Torxs sockets, Sears has them in their tool dept in sets or individual sockets. BMW uses lots of Torxs head bolts. I replaced the speaker bolts with philips head versions when I was reinstalling as the Torx heads are a nuisance on small screws like this. Once you've removed the screws holding the shelf panel to the speakers, remove the interior DSP trim panel ( the front of the two piece rear shelf panel ), and then remove the rear shelf panel, too ( clips under the vents in that panel at the rear edge directly below the back window ). With those panels off, you can now remove the push pins, wiring plug, bolts, etc that hold the entire DSP/ Speaker assembly and put it in the trunk. Be careful not to puncture the factory speakers once you have the grilles off or while you remove the DSP assembly!

With the DSP out of the way, you can lift up the foam insulation pad and can now get at the upper strut bolts ( finally getting to the strut bolts which is why we're doing all this!) Use a 13mm socket to remove the nuts. You can now go back to the wheel well and leverage down the rear control arm enough to remove the rear strut assembly. With the car securely on jack stands, you will have to push the control arm down to get the strut assembly out. Put a cloth at the fender lip to avoid scratching the paint as you remove the strut assembly.

Additional note for the driver's side rear: you'll have to remove the forward bolt that holds the fuel evap system and move that container assy out of the way enough to remove the strut. These two pic's show that assembly...both the upper cannister bolt and the rear cannister and lines.

Here's the comparison between the loose H&R spring ( red )and the factory assembled stock spring & strut....all this for just a 1/2" of lowering! Now all you do is reverse these steps and reassemble your car. Make sure you do a 4 wheel re-align! As you can see, replacing the rear springs ( or struts ...) isn't hard: it just takes more time and patience compared to the fronts.