The installation of a trigger guard on an AR-15 is probably the step that most often leads to failure for a novice assembling their first lower. Left unsupported, the ears that hold the roll pin which secures the rear portion of the trigger guard can easily break off.
Gently tap the pin into the top ear. Do not drive the pin too far or you will not be able to insert the trigger guard. Place the rear of the trigger guard between the ears and, using a steel punch, continue to tap the roll pin in.
Make sure that the lower ear is supported when driving the pin. Continue to drive the pin until it is flush with the ear.
If you are using a standard trigger guard, depress the detent on the front of the trigger guard and slide it into place. We chose to install a MagPul MOE synthetic trigger guard, which is held in place with a small hex-drive set screw.