It requires patience, but just keep telling yourself "This is not impossible to do".
Probably recommended that you get new drive shaft bolts before you start, though some folks claim to re-use theirs with the addition of
a bit of non-permanent LockTite. I am a "use new bolts" advocate, myself.
Put the bike on the centerstand.
Remove your panniers - if you have them. Undo the clamp at the back of the transmission that holds on the front part of the boot and slide the boot back so you can see the attachment bolts holding on the drive shaft coupling and transmission output flange. Locate that small, thin 12-point closed-end wrench from your toolkit for undoing the driveshaft coupler bolts. Put the bike in gear and step on the rear brake pedal - this should hold things solidly enough to loosen each of the bolts. Now, I have never replaced the boot without also having the swingarm bolts removed so that I could push the swingarm back, it might be possible to do without undoing the swingarm, but I haven't done it myself. To get the swingarm to slide back, disconnect the rear brake actuating lever, loosen (but don't remove) the upper shock absorber bolts, and pry off the plastic caps from the swingarm pivots. you will need a thin-walled 27mm socket to get in to loosen the locknuts on the swingarm pivot bolts. Before you undo the swingarm, measure the spacing on the inside edge of the frame and swingarm tube ends - this is so that you can get the swingarm back on there properly centered - I'd recommend using something with better precision than a tape measure, but this picture gives you an idea, anyhow - measure both sides.
Be sure that you "block up" the bike with some pieces of wood or something under the engine sump, and I use a jackstand that I place
under the crossmember at the lower rear part of the frame, behind the transmission. These keep the bike from tipping front or back while
you've got the swingarm partially disconnected and allow you to swing it to the rear to provide clearance to remove the old boot and install a new boot aththe drive shaft/transmission coupling.
Some thin walled 27mm sockets can be found at auto parts stores or maybe Sears that will fit without modification, but often one has to at least grind off the rounded edge of the socket for it to fit securely in the swingarm pivot housing - you don't want to bugger up the locknut.
BMW sells a special tool for this if you don't have it in your toolkit, or you can find a socket on your own and grind it down if needed, or there are folks on the internet (Ed Korn, among others) who sell modified sockets for this purpose.
So, loosen those 27mm locknuts and then use the Allen key in your toolkit to remove the swingarm bolts/pivot pins. You can then
push the swingarm toward the back - it will pivot on the upper shock absorber bolts that you loosened, and you can stick in a 2x4 piece of wood between the front of the rear tire and the fender to keep it held back (bungee it in place to it doesn't fall out while you have your fingers in a potential "crush zone" installing the rubber boot). you should then have enought clearance to get the old boot off and new boot on without cutting the new boot. You may want to use new clamps, or re-use your old ones if they are in good shape.
Clean and regrease the swingarm bearings while you have the pivot bolts out, and reinstall things basically in the reverse order. You want to set the preload on the swingarm bearings per BMW's instructions, but make sure that you have the swingarm centered properly (using those measurements you took earlier) before you set the bearings. Don't forget to re-tighten the shock absorber mounting bolts when you are through !
