There is support material built into the model for easier printing, the material is on both sides of the mouse buttons and under the arch for the wire. I recommend using an X-acto or some other thin blade to cut it off. The "Buttons" should move freely without clipping/hitting the rest of the frame.
The two buttons are mounted with 2 M2 x 20mm machine screws and nuts, solder the wires to the switch at a 90 degree angle so there is enough room.
Board and Lens
The lens falls into place in the recessed rectangle in the middle then the sensor board goes on top of that, make sure the yellow kapton tape is removed from the sensor and that the sensor falls into the lens correctly. Then just screw the board down.
The bottom baseplate tab slides into the top shell and the shell is secured with a M2.5 machine screw