I accidentally popped a trigger off, and both L1 and R1, because I skimmed and didn't read this step. Fortunately it's not hard. The tricky part with the triggers is that there's a little spring on them. You have to press the side of the spring that's pointing away from the trigger against the sloped surface of the controller (look at the other trigger for reference, if you only popped one off). Then you just slide the trigger on until the "handlebar" bits near the center click into place.