The firing pin is pretty straightforward to install. First, stick the firing pin spring on the pointy end. The firing pin spring is tapered on one end; the skinny part goes on the firing pin - it should take a bit of effort to push on, and stay put once installed. Insert the firing pin safety lock, half-moon side backwards, and push the lock in while inserting the firing pin. The scalloped sections need to be up. If the pin is in the proper orientation, it will snap into place.
The trickiest bit is re-inserting the roll pin. The pin needs to be in the forward position (as through striking the primer) - push it forward with a punch or something. The roll pin will fall into place up to the striated section, at which point one merely needs to bludgeon it into place. (The striations came off in tiny metal beads - this really is a one-use item.)