I had to do this job twice when I was 'freshening up' the bike - very frusterating to get it all back together, push the starter, and watch oil stream out from under the trany! I mistakenly tried to push the rear seal over what I (and the BMW mechanic that was helping me) thought was the crank shaft, but what turned out to be a removable collar.
Check that everything has been removed - my Haynes manual was not to specific about this, which partly lead to the above mistake. Make sure you measure the installed height of the existing seal before removal (how deep it sits in it's seat). I used a section of pipe to drive the oil seal in to position. You need to find the right diameter that will press on the hard outer shell of the seal. I can't remember the size I used, but it was fairly common - 3 inch I think. The newer type seals should be soaked in oil for 3 hours before installing. Good luck!