Start by folding the two halves inwards towards each other, to shrink them.

Hook the IR Ring shield onto it's spot just below the trigger, and then fold it down onto the two inner IR Rings, while still folding them together

You may have it where one of the halves didn't click into place, push, starting from near the main part of the controller, and slowly slide to the front, it may take a bit of force

Afterwards, you should have the IR Ring back in place.