You need to get FP block plunger, FP block spring, and FP return spring all in there correctly, then...
I run a steel 1/8 punch through the opposite side to hold the FP in place, acting as a sort of slave retaining pin, then I use a steel hollow-end punch and a steel-headed hammer to get the FP retaining pin started. The FP needs to be held in place, either from the rear, or with a punch from the other side like I do, or else the FP retaining pin will collide with the FP... so you don't want to just beat on it if it gets stuck like this, or else you can damage the FP, the retaining pin, or both.
Once the retaining pin is in far enough that it's holding the FP in place on its own, then I usually push it most of the rest of the way in with my vice, then use a brass punch to get it past the slide serrations, then finally back to the 1/8 steel punch to get it into the exact right position.
You don't want to hit it with a usual steel punch until it is already in the slide or else you can mushroom it out.
