Pew Pew Pew
Step 1: Begin by 3d Printing the Files
Download and print the two files
All you really have to do is print the files and insert the trigger into the back of the body and attach a large rubber band (yellow in the pic) from the loop in the trigger to the front.
Note*- The body is designed to be printed upside down and the trigger on it's side
Once you've got it printed just load it from the bottom notch to the top and FIRE - the rubber bands should index up as you pull the trigger giving you up to six shots.
You may have to knock the corners off the notches to get the bands to "feed" up the back of the gun correctly - I also found if you scale it to 125% of the original size it works a bit better too.