You will need:
--A pen that clicks
--Slight use of your mind(if that's a problem stop here)
Step 1: Taking it apart
The picture below shows the pieces of the clicker. When you take apart your pen you can find them.
Step 2: Putting it all together
To make the gun, spear the gripper with the ink tube right through the middle. Then put the spring on the tube and twist it until it cuts into the gripper, which secures it when you fire the missle. You can add a rubber band on the back, and secure the missle with it when not in use.
Below is the gun.