If the spring/strut/plate are still assembled, you insert the tip of the strut into the hammer, rotate the botton into place in the gripframe, cock the hammer to release whatever you locked the spring with, and you're done.
If you disassembled the strut and spring, that's another story. With someone helping it's safer. lock the top of the strut in a vise, slip the spring on, place the retainer plate, wings toward the strut's bottom (down installed) and using pliers or such, pressing on the plate, compress the spring until you can get a 1/16 dia pin or small allen wrench into the hole in the strut, the release to lock it together. Then insert the assembly into the gun as above.