Step 7: The trigger and the trigger guard

Get a thin but long(ish) piece of cardboard. Squish it between your fingers to flatten it a bit.
This will be the trigger guard. Don't make the length exact so you can actually glue it to the grip thing and the bottom of the long part. Anyway now, Glue it to the grip thing, folding it a little so there is some support.

Make a trigger, just a piece of cardboard shaped like a half crescent. Glue it to the bottom of the long part right before it meets the trigger guard. When it is dry, glue the rest of the trigger guard to the long part.