Just pulled mine last night. Beforehand, I did a lot of searching through posts on this subject. I found it easiest to jack up just one side of the car, front and rear, with good jack stands and
chock the opposite wheels as you will need the car in neutral with the parking brake off.
I believe your driveshaft will have the same index marks on the two portions to reorient to the same position. They appear as a single > on the front and two< on the rear (Look at the flats on the mating ends.) But marking with something more visible would be easiest.
If you're just doing the front one, first you will need to lower the exhaust but it can remain attached at the front end. Then remove the cross brace for the transmission support and the tunnel cover. The center support can remain but loosen it to make it easier. Loosen the clamping nut to enable compression of the drive shaft then the bolts for the disc can be pulled. Note where the bolts pass through the "star" of the balancer. Compress the shaft and swap them out. Also, note that there is a front side of the disc and the ferrules of the disc fit into the transmission flange and driveshaft flange (very important!) Use all consistent hardware.
Unless cost is an issue, you may find it worthwhile to go ahead and do the center support and bearing along with the rear flex disc as you will have done 80% of the work already. It's not a difficult job but can be pretty grimey if you've dealt with fluid leaks in the past. If you decide to clean it up a bit beforehand, let it dry overnite to avoid the constant drip!
Here is an excellent wikka
http://www.peachparts.com/Wikka/W123DriveShaftSupportBearing
Also:
http://www.peachparts.com/shopforum/showthread.php?t=235575&highlight=flex+disc
http://www.w124performance.com/docs/mb/W124/driveline_balancing.pdf