Replace the spacer rings. The brass rings go on the bottom, the thickest black ring is on top. The rings fit onto a tab that keeps them aligned. Make sure the holes line up with the screw holes in assembly below.

Drop on the mounting plate. The long leg goes inside the lens and into a hole in the central lens assembly; it's easy to see.

Secure the mounting plate with four screws. The shorter screw is directly opposite the red dot.