Only really one thing to know.
The firing pin spring pushes the pin away from the breech. Drop the spring in place, slide the pin forward and down. Install cross pin.
Ok, one more thing. If you don't install the firing pin retaining cross bin, it'll work just fine, until someone dry fires it a couple times, and peens over the chamber edge. The cross pin also limits the forward travel of the pin. Just so you know.
Install the cross pin.
Cheers
Trev