I grease them up with synth silicone. Then I hold the rod end straight up (with the allen wrench hole down}. Place the o-ring on top place my index finger on top of it and mash down as I rotate my finger around the periphery of the rod, slowly getting one side of the o-ring down in the groove. Then work the other side. Then I have to squeeze that o-ring out of groove 1 and down onto groove 2.
Or, you might take a toothpick and place it along side the rod parallel to it, drop the o-ring down on the toothpick and roll it down the toothpick and rod to the second groove. The toothpick should keep the ring out of groove 1. Repeat for the second o-ring. Haven't tried that toothpick method but thinking it might work.
It's a pain in the butt, that's for sure! Good luck.
Y