Remove one PH00 screw that is securing the rail to the back half.

The FR button can be removed by placing a lever/spudger under the circled area.

The rail connector can be removed once the yellow circled screw is removed.

The button that releases the buckle lock can be removed after the rail and goes in the hole circled on the back half of the controller.

The one remaining screw (not marked) does not need to be removed as it only secures a bit of plastic.