Replace the plastic panel by dropping it into place then firmly pressing down on the top of the panel until you hear the clicking of the little clips inside.

Make sure it is properly reattached since there are a few clips inside which help to keep the panel in place, you don't want this coming off again!

REPEAT: This can be done in the exact same way for both triggers, since they are both under an identical plastic panel and you can do this exact guide for both sides of the controller.