What you need:
-Two 30 cm (12 inches) rulers. One of these needs to have a groove down the middle so the pen doesn't fly off the worng direction.
-A fairly large elastic band. (I used one the postman dropped.)
-Some tape (I used masting tape)
-An empty drinks bottle. (I used a Lucozade bottle as it felt quite ergonomical whilst firing.)
Step 1: Forming the Main Body
In this step I shall show you how to create the main frame of the crossbow.
-Firstly lay your two rulers across each other making sure the grooved one is vertical and over the top of the other one. ( A bit like a crucifix so the two sides of the vertical one aren't equal.)
-Now tape the sides together around so the groove isn't covered by tape.
Step 2: Attaching the Handle (the Bottle)
Step 3: The Elastic Band
This is probably the hardest part to explain as the band must be put on correctly or risk having it fly off as well as the pen.
-Firstly, Pull the elastic band up the longer side of the lengthways ruler.
-Secondly, pull it under the bottle.
-Finally hook it over the end of the ruler
If you put your elastic band on this way it will not fly off the 'crossbow'.
Step 4: Now Go Kick Some Butt!
This is my first Instructable so sorry if it isn't very good.