You'll need the pieces shown in the picture as well as 3 'standard' elastic bands or some reasonable equivalent. If the gun snaps often you can superglue the parts on. So let's get started!
*The trigger should fall down due to the elastic bands.
1 'hubcap' brown
1 black joint
2 light grey
2 black 'square' joiners
6 blue (4 when using revised version)
If in doubt all will become clear in the following pictures.
Step 4: Build Trigger
Step 6: Main Barrel
EDIT: Please look at note (yellow box surrounding blue connector) the gun works better without this blue piece - sorry for inconvenience.