Lay the base on your workspace, "beams up".
Take one vertical beam, and glue the insides of the loose flaps. Stand it near one end of the base, with the flaps astride the beam (see photo). Press the flaps into place for a few seconds, then repeat with the other vertical beam. Before the glue has a chance to dry properly, thread the cocktail stick through the holes you pierced yo make sure they are all in line (you should be able to slide the second beam a little bit if you need to, before the glue dries).
Take one of the diagonal beams and glue the insides of all six flaps. Set it against the side of one of the vertical beams, with the four narrower flaps astride the vertical beam and the beam of the base. The end flaps should be pressed into place against the vertical beam and the base beam.
Repeat with the other diagonal beam on the other vertical beam.
Hold the firing arm between the tops of the two vertical beams (make sure it's the right way up!) and thread the cocktail stick back through the whole lot - both vertical beams and the arm.
You need to give the joints a little time to set again, but otherwise you're finished.
Load up the bucket with the ammunition of your choice, and LOOSE!