Place the auto sear spring back in place as shown in the first picture.
Insert the pin and brass spacer halfway in, then place the auto sear in as shown in the second picture. Push the pin all the way in. The spring leg that sticks out should point towards the FRONT of the gun.
Put the trigger back in place, with the front of the striker resting on top of the brass spacer. Be sure to install the trigger spring properly. Place the hammer over the striker and install the pin through hammer and striker.