If you are lucky you are looking at 30-60 min of work, if not.... well. You will need a number of tools (ratchet, universal joints, sockets and extensions, etc) for this job. Under normal circumstances and with the mounts just slightly collapsed you need to disconnect the exhaust pipe going from the left side header to the main pipe. Its the one that goes across the transmission. Then you disconnect the two lower motor mount retaining bolts on the crossmember of each side and loosen up the fan shroud and place it over the engine fan before you jack or lift the engine up. If you dont have an engine lift, put a piece of wood on a hydraulic jack and carefully raise the engine placing the jack under the oilpan.
If the upper bolts are not seized up through excessive vibration, you can now loosen the upper bolts and take the mounts out.
There's a heatshield on the right side protecting a steering balljoint, that will have to come off too (the heatshield) to make room for the pretty fat mounts to come out. Jack the engine up as high as you can without damaging the throttle linkage and they will come out rather easily.
In case the upper bolts cannot easily be broken loose you will have to disconnect all four bolts on each side that attach the engine carrier arms to the engine. Now you've got a loose enginemount-enginecarrier combo in there. You will have to wedge them inplace somewhere and brake the upper bolt loose.
It will be more difficult on the left side but with patience it can be done. If the mounts have been in there too long and are completely collapsed you will have to go through what I just described, if not it should be fairly easy. Once you have them out , putting the new ones in is the reverse of the same procedure, only you need to line up the mounts and the lower bolts after you have connected the mount with the upper bolt to the engine carrier (then re-attached to the block).
Even the worst mounts can be replaced. Its just a matter of persistence, heavy cursing and brute force. I once spent almost an entire day on two completely seized up bolts on a M119 in a W124. But in the end they all come out, one way or another and the resulting smoothness of the engine later on is more than worth the trouble. For some reason these mounts seem to go bad pretty consistently every 50k-75k miles. My 400E has 120K down and the mounts have been done twice now. The ones on M103 engine W124's seem to last quite a bit longer.
Good luck!
F.J.

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F.J.
81 300 SD grey/palomino 168K miles
84 190E 2.3 black/grey 64K miles (wrecked)
85 190E 2.3 maroon/palomino 92K miles
88 300E desert red/palomino 204k miles
(made to look like a '94-95)
92 400E desert taupe/creme beige 120K miles
(converted to 94-95 E420 looks)