Before ...
One of the things that we afficionados of older BMWs face is that the cloth upholstery material used in the cars, although gorgeous and soft and lovely when in good shape, sure turns into an ugly mess when it finally wears out. One month your seats look okay, the next month they look like wild dogs have been ravaging them. And there are few things that make a car look more like a beater than having shredded seats.
But they are fixable. If your seats are just starting to get a few loose threads, you can save them (for a while) by using "Sew No More" mending adhesive (get it from fabric stores). If it's just your driver's seat that's worn, you can get a matching passenger's seat from a wrecking yard (the passenger's side generally gets less wear), and then swap all the reclining mechanisms and seatbelt latch hardware over to make a new driver's seat. I did this for my old 320i and it worked pretty well.
But when I got my current 325, the seats were an enigma. They were the semi-rare factory Recaro-style seats, so I wanted to keep them, but they were both completely shredded. For $45 and four evenings of work I made them both look and feel like new. Here's how (Note, this works for 1976-1991 seats, I don't know about the newer ones):
Before ...
... After
Go to the wrecking yard, and get the REAR seat out of a car with matching upholstery. Be sure to get it from a recent wreck that hasn't had too much sunlight through missing windows (it's amazing how fast the material degrades in unfiltered sunlight). I got really lucky, and found a very recently t-boned 318i, which had been hit just hard enough to crimp the rear seat frame, rendering it useless as a seat, so I got it cheap (the cloth was just fine).
If you don't have a sewing machine, borrow one from your your girlfriend, wife, or mother. Do NOT tell them what you are going to use it for. Tell them you have some pants to hem :-) Ask them where the spare needles are, and how to change the needle. Also borrow their emboidery thread and needles, plus their "seam ripper".
On your donor rear seat, you'll find that the cloth is held in place by A) a bunch of sharp metal tabs, bent over, and B) some "tuck threads" going through the pads and tying onto a metal rod (these provide that nice factory-fit look. They have a plastic backing which you can see against the horsehair pad). Using pliers, unbend all the little metal tabs, and carefully lift the cloth off of them. You can now lift the cloth off from the pad. When you come to the "tuck threads", just cut them.
On the seat you want to fix, do the same thing, unbending the tabs and cutting the tuck-threads. If you are repairing the seat-back, you'll need to mechanically disassemble the seat-back from the seat-bottom first, and it's also helpful to do this even if you're just doing the seat bottom (though I imagine you could do the job with the seat intact). Taking the seat back off is a mechanical job. There are screws holding the seat-back's rear cover in place, and holding the plastic covers on the recline mechanisms. Remove the screws and the seat-back tipping handles (the tipping handles just yank off, then the surround for the hole sort of lifts and yanks). A mess of big screws and circlips and stuff hold each recliner mechanism to each side of the seat. It's pretty easy to figure out.
The Recaro-style seats are actually modular, so you don't need to disassemble the frame. You just need to remove each pad from the seat by unbending the little wire hoops that hold the pad onto the frame. Don't lose them, as you'll need 'em to reassemble the seat. When I did the bottom side bolsters, I didn't want to bother dissassembling the seat frame, so actually cut a 3" slit to allow the material to come up from the pivot. I also had to have my driver's seat frame welded, as fourteen years of people flopping into it had broken the frame.
Now you should have the shredded cover from off your seat, and the intact material from the donor seat. Using the seam ripper, carefully separate the shredded panel(s) from the good parts (you do have SOME good parts, right?). Keep a mental track of how all the seams go. Then do the same to the donor seat, so you just have nice, unsewn cloth (don't separate the outer cover from the cotton backing though).
Now you just need to cut out the shapes you need from the spare cloth. Squarish panels with symetry are pretty easy, but when doing side bolsters be REALLY CAREFUL that you don't accidently cut out the weird shapes with the old and new material face-to-face, or you end up with a mirror-image panel, which won't generally work (I nearly ran out of material when I made this mistake).
Now the tricky part. You've got to sew the new panel in where the old one came out. Seams are sewn with the workpiece "inside out" (the bits you'll see facing each other on the inside, and the underneath parts that you won't see facing the outside). If you've never sewn before, practice with some old jean material first.
The seat cloth is pretty thick, so when you start sewing it, you'll probably need to crank up the thread tension a little to avoid the thread bunching on the bottom. If you are having to sew through the nasty plastic "piping", be aware that the sewing machine ain't gonna like it. You may need to carefully hand crank the machine to avoid all sorts of jamming and needle breaking. It's still better than sewing completely by hand.
After much swearing, cursing, and broken needles, you should have a new cover that pretty much resembles the old cover, except without the shredding. Use the embroidering needles and thread to sew some new "tuck threads" onto your cover at the appropriate spots. Try to sew these through the cotton inner cover and seam, but not out through the main outer cover.
Now just stretch the new cover back onto your seat frame, threading the tuck-threads through the pad. Working from front-to-back (or vice-versa), tension the material and stab it onto the metal tabs, then bend them all over (if you've got Recaro-style seats, you must also replace the metal hoops ). Tie your tuck threads onto the metal rod, making them tight enough to give a little pucker, and then reassemble your seat. Voila! Your car looks like the finely-crafted machine it should be.