I recently replaced the rear arms on my 71 Catalina,
(1) jacked the car an put the frame on jack stands
(2) removed both rear wheels
(3) removed the 2 bolts holding the arms in place. (this can be difficult - depending on how rusty the car is, the bolts can rust to the steel inner bushing of the rubber control arm bushings. If that happens the only solution I have found was to use a sawsall and cut thru both sides of the bolts. And the bolts are Grade 8 and hardened so it can take awhile. Hopefully you don't have that isue.)
(4) install the front bolt on the arm
(5) place a jack under the rear axle tube and lift the axle to a location that the rear bushing mounting hole lines up with the axle brackets.
(6) repeat 3 more times.
Remember safety 1st, good jack stand on a flat surface.