Place rear wiring into stock and slide the gearbox into the receiver. Do not let the selector pieces get out of place.

A good way to know if the selector parts are aligned correctly is by keeping them in the safe position. If the trigger is able to move after the gearbox is in position, something got out of place.

When gearbox is in position the hole of the largest selector piece and the hole of the receiver should line up.

Now place the circular piece into the hole. If the selector lever is still attached to the other piece then just add them both. There is a small slot on the round piece that correlates with a notch of the selector make sure they fit together.