Jack stands for the rig. Jack and stands for the axle. Tight shackle bolts. Placing the spring and jamming the bolts through the shackles is not really a one man job. Unless a person walks on their knuckles.
Save time. Soak the spring U bolts for three days with Liquid Wrench. WD40 is useless.
The axle MUST be raised or lowered to clear the spring slowly and carefully. This is for the benefit of the spring on the other end. Careful with the trailer brake wiring.
I had a set of jackstands that were modified. I had a piece of 2" angle iron welded on each stand's pad. Super sturdy axle support.