you need to have both left and right side wheels off the ground, with jack stands supporting the frame.
That's it. Raise the rear end and sit the frame on jack stands making sure the wheels aren't resting on the ground. If the rear end is just hanging there without any ground contact then just unbolt the ubolts to remove the spring plate. The axle with just sit there on the springs, no worries.
One thing I do when I don't have enough wide pieces of wood to stack UNDERNEATH the jack stands... I raise the rear end up at the diff and set the jack stands under the frame rails. I then remove the tires so when I lower the axle back down to just hang the jeep isn't as high in the air. You might not be able to get an impact under there but if you just but you can just go higher unless your using a breaker bar.
When I did my lift, I gathered 4 jack stands. 1 for each frame rail and 1 for each side of the axle. I jacked up the rear end, pulled the tires, then lowered it on to the frame rail stands. Once I had the frame supported, I slipped one under each side of the axle to support the axle. Now the axle can't come down too far and the frame is supported. From there the only thing that can move is the spring.
I removed the shocks first, so I didn't over extend them, then dropped the springs. Since you're just doing the plates, you can add 2 more jack stands - front and rear of the spring plates - to keep the springs from dropping too far.
Safety first, second and third.
1996 White ZJ
1987 Maroon YJ (sideways build)
1996 Forest Green XJ