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Picked up a set of Whiteline rear subframe inserts after reading the thread in suspension subforum. Decided not to go with the full M3 set due to labor involved. It looks like these should actually do a really good job anyway. This is a DIY for a home install with no lift. Shouldn't really matter, though.
Special tools needed: E18 female torx. Had to hunt for one of these. 16mm ratcheting crescent wrench would be super nice, but not required.
Here are the inserts before install. This is the rear pair with a subframe bolt sticking through to show approximately how they sit when installed. I think I actually got them rotated 90 degrees out, but you get the general idea. The extra single piece on the top left is the other side rear bottom insert turned bottom up.
To get started, chock your front tires, lift the rear of the car with your jack on the differential. Put your jackstands in place. If you don't have any I suggest getting some BMW jackstand adapters. Take off the rear tires, it will make life a LOT easier especially if you have no lift.
Leave the jack in place under the differential as it will keep load off the subframe bolts. Take out the two rear subframe bolts with an E18 bit (18mm female torx). They should come out by hand after you get them started.
Back to the front, you'll have to take two 16mm bolts out of a extra diagonal bracket that connects the front of the subframe to the chassis.
The bolts are hidden and a pain in the ass to get to. You can use a 16mm crescent wrench or take some of the plastic off. This is immediately in front of the rear tire.
I think you really need to full remove them to get enough droop to fit the inserts in.
Once those two bolts are out on each side, loosen the front subframe bolts. You'll need an extension to fit up into the mount bracket. Same E18 bolt head as the rear subframe bolts. I'd loosen them all the way then put them back about 3-4 full turns. With a jack under the rear you might be able to just remove them, but I left them in place to help keep the subframe aligned.
Now back to the rear subframe bolts. Lower your jack until you have room to place the upper rear subframe insert in. Grease it up (I lost the grease that came with the pack, just used some general purpose grease), and insert it. The rear upper has the skirts facing inside and outside, not front to back. It really only goes in one way, but pay attention.
Once both sides are in, grease up the rear lower, push it in (there are "keys" on the bottom outside edge that match up with the subframe so you can rotate it correctly). Jack the diff back up so you can get the rear subframe bolts back in 3-4 full turns on both sides.
Back to the front mounts, fully remove the subframe bolts. Lower the jack off the differential. There should be just enough room to slide in the inserts. I started with the top. This one was tricky to correctly align. I believe the skirts go front and back, and the block that hide under the rim go to the sides. Sorry, not 100% sure, forgot already, but you should put your finger in there and try to match the shapes correctly.
I think this picture shows the skirts towards the sides but I ended up installing it rotated 90 degrees from this picture (this is passenger/right side). Sorry, I don't recall exactly how they have to be rotated. It's not obvious, so jam your finger in the top and see which way fits right. The front upper insert is the only one that is not obvious.
Again, grease them up, and you'll have to kind of jam them in. Be very careful that the fingers do not get bent over. Make sure then go straight into the bushing. I had to jam them in a bit due to clearance as the front of the subframe doesn't like to sag enough to make it easy.
The bottom front insert is easy to align because there is a "key" that matches with the subframe as seen here, just like the rear bottom inserts (bottom front passenger side subframe insert here):
Once in, put your front subframe bolts back in, tighten, then put the two 16mm brace bolts back in, and if you removed the plastic reinstall. Go back to the rear subframe bolts and tighten those. Try to keep an eye on the outer exposed lip of the inserts so they do not fold under. I had to use a small flathead to keep the lip from folding under itself on a few spots on the top inserts, both front and back.
Lower jack, re-torque all your subframe bolts, put your wheels back on, jack the car back off your jackstands, remove jackstands, lower car, and you should be good!
I may go back and check torque again later to see if the subframe settles any.