It's a full mini crossbow! All you need is some string and its just like a normal crossbow.

To assemble all you have to do is put the trigger in the the hole and insert the pin through the two hinge things so that it moves when you press on it. Then tie the string through the 2 holes on the end of the crossbow arms. (you will have to adjust the size of string according to your specs) Then to arm it just put the bolt on the end of the trigger piece and pull the string back onto the bolt. Then just press on the trigger to fire!