Just did my RRC yesterday. There is a gasket kit with rings available from LR. I found it easiest to place a flat piece of metal over the ring with the upper seal in the ring and then use a pair of channel locks to gently press the flat bar until the retaining ring snaps into place. Make sure the rings are not bent or you will never get them straight. Also having a few extra around if a good idea till you get the hang of it. Another tip is use compressed air to pop the pistons out of the caliper.
72 Series III 109 SW Diesel, 03 RR, 98 Porsche 986, 01 Porsche 986 3.6 L