The first time I replaced a sideplate assembly was by trying to keep everything in place with a brass punch, but things didn't go so smoothly and I wound up having to put all the pieces back in. I found the Youtube video in the earlier post to be helpful with this, but Fastbolts method is the one that really worked for me. Punching the sear pin out with a plastic hammer and using a flat piece of wood to snap the sideplate legs back on is the way to go.

It can be daunting at first, but after a few times re-assembly gets pretty easy. If you ever decide to try and fit/adjust a sear release lever, it will pretty much guarantee mastery of sideplate installation.