Really, it will be best to watch the video for this. I will try to explain some of the tricker parts, but it is pretty straight forward if you watch the video.
I used a good quality wood glue, and I just squirted some out on a sheet of paper. I then took a tooth pick and scooped a little glue up and smeared it on the pieces I wanted to glue together. Then I just glued them together.
I didn't use any fancy clamps (but you could if you have them), the fanciest I got was some rubber bands to hold the barrels in place while they dried, the rest was just gravity.
Vertical String Guide - I started with the gun stock and the string guides. The vertical string guide goes right up against the front of the stock. It also helps to strengthen the area since you had to drill out the hole in the stock for the rotor axel. Once this is glued in place, it should bring a lot of strength to that area.
Horizontal String Guide - In the video, I glued the horizontal string guide right up against the vertical string guide. I wish I had put some space in between these 2 parts though. It still works on mine, but I can see it would have worked better if the horizontal guide was placed farther back toward the angle on the stock.
NOTE: This is also where you can make this either a right hand or left hand rubber band gun. But choosing which side the horizontal string guide will feed the string to, will determine if you wind the crank with your right or left hand. I am right handed so I placed the horizontal string guide hole on the right side of the gun stock.
Barrel Axel - Next I took the barrel axel, rolled the end in a bit of glue, and then put it in the hole of the gun stock. I tried to make sure it was sitting as straight as possible before I found a place I could lay it where it wouldn't be disturbed as the glue dried.
Hand Crank - Next I glued the 2 dowels into the hand crank piece. I needed that dried before I could glue the assembly to the gun stock. Just eye ball it and get them as close to straight as possible. Make sure and put the axel in the hole at the larger end, and the hand crank at the hole in the smaller end. I marked my dowels with pencil after I cut them to make identifying them later easier.
Barrel Rotor Assembly - I used rubber bands to hold this together, then I put glue into the front end and let it dry. It is easier to see in the video how to do this, it's much more difficult to explain. Don't glue the rear rotor (the one with the 1/4" holes in it) as you will need to space this once more of the gun is assembled and then glue it. Again, this is shown in the video more clearly.
LET ALL OF THESE PARTS DRY AND SET FOR AN HOUR OR TWO
Barrel Axel Spacers - For this part I placed the barrel axel spacer on the axel, then I placed the half glued barrel rotor assembly on and I just used business cards in between the spacer and the gun stock until I could safely spin the barrel rotor assembly without the 1/4" dowels hitting the string guides (I hadn't glued the 1/4" dowels in place yet, I just placed a few in there to size it up). For me, the magic number was 8 business cards, but this can change depending on how you assembled the first parts of your gun. Again, if you watch the video, you will see how I did this with business cards. Once I had the space figured out, I just put a bit of glue on the spacer and set it aside to dry.
Rear Barrel Rotor - Now that I had the spacing figured out for the barrel rotor and spacers, I was able to glue the rear rotor in place. Just like the front rotor, I leave the rubber bands on and just separate the barrels from the rotor enough to slip a little glue in between them.
1/4" Dowels - I just rolled the ends of the 1/4" dowels in glue, and stuck them in each hole. These need to lay flat against the barrel (any space between these will allow the rubber band to slide under and behind the barrel and will cause mis fires). If the dowels were fitting right up against the barrel, I just used a rubber band around the 1/4" dowel and the barrel to make sure it dried making contact.
Hand Crank - Next I finished the hand crank. I put a spacer on the axel, then pushed it through the gun stock, and put another spacer on the other side. I moved the axel in and out until I found the proper length where the hand crank wouldn't hit the string guides when winding. Once I found that spot, I marked it on the axel with a pen, and then I cut it where I marked it. Then I got my business cards out again, and put a few cards in between the spacers and the gun stock on both sides. There just needs to be enough space so that the crank can move without being pinched against the gun stock. I did 4 cards on each side, but that's probably a bit much, 2 would have been enough and it would have made less wobble, but 4 worked ok still. Then I just put a little glue in on the spacers to get them secured, and set it aside to dry.
NOTE: If you have chosen a left handed configuration, you will need to make sure the hand crank is placed on the same side as the string hole from your horizontal string guide.
Barrel Rotor Assembly End Spacer - Next I finished off the barrel rotor assembly by using the business cards to space them, and then glue them in place. I had to cut my business cards to fit into the smaller place. Again, it just has to have enough space between the rotors and spacers so that the assembly can spin freely. I used 4 cards, but 2 would probably be sufficient and create less wobble. Once cards in the back and front of the barrel, I glued the end spacer in place.
After that, just let all of your glue dry and you're ready to add the string.
String - Take one end of your string and thread it into the rear barrel rotor, tying a knot of the inside of the barrel assembly so the string can't pull back through. Then thread the string through the vertical string guide and the horizontal string guide, and finally wrap it around the hand crank axel and tie it in a knot around the axel. Apply a bit of tension on the line, simply wind the string up with the hand crank until you have all of the string wound around the axel. You're all done and ready to load your gun and start shootin' pardner!