Snap ring pliers work the best for them. Other option is to use 2 flat screw drivers, hold 1 hole down and use the other to work the ring into the gap and around. Little bit of pressure, but do not force it too much.
Edit - Sorry, totally missed it the first time, you are referring to the gauge locking ring, it isn't a snap ring, just a 3 sided plate which should slide into the groves, I use 2 screw drivers and get one side almost in and spin it with another screwdriver and a slight tap from a hammer. I also apply some downward pressure to get it to seat correctly. Another tip is to make sure the bottom seal is warm so it compresses when installed. Hope this helps.
85' CJ7 Laredo, 383 GM, Turbo 350, NP 203. SOA, 2" frame lift, 34" TSL swampers, Dana 30 open, AMC 20 w/ detroit locker, 4:10's, 1 Piece Mosers, 35" boggers to abuse when needed.