On the rear…
Secure the lever into the pivot with the metal pin.
Place the pivot onto the back of the IR sight housing so that the pointy end sits in the hole on the housing. Make sure it can move freely in this hole, and that it catches the IR Sight inner assembly without any snags.
You may need to trim the end of the latch ever so slightly to make sure it has good operation.
Glue the pivot to the housing.