Building up a bolt completely from scratch parts, attempting to insert the safety selector into it's mating part, I can get the small plunger on the selector to move/compress with a small tool/driver but can't keep it depressed enough to slide it into it's mating groove...just not a lot of room to work with (in contrast inserting or removing a magazine cutoff is much easier...more room to work with).

Is there a special tool to do this (which of course I don't have!) or some other clever technique?? Thanks!

OK, thanks. Followed your suggestions, used jewelers driver & managed to finally get it. Was originally holding it all in my hands too but finally put the flat faces of the bolt sleeve in a vice (cushioned) to get max leverage on the selector & plunger, could never really get the plunger far enough in to just push the selector on by hand but got it close enough to tap gently with a rubber mallet & nudge it over the lip.

The project piece is on the right. Appreciate it!