How do I set the weapon up

Hint 1 of 7:

First you need to assemble the catapult.

Hint 2 of 7:

GET CATAPULT ARM  and then USE CATAPULT ARM WITH CATAPULT.  +2

Hint 3 of 7:

Lower the arm for firing. MOVE LEVER on left side.     +1

Hint 4 of 7:

You must be in front of the catapult to move the lever.

Hint 5 of 7:

You need a missile for the catapult. GET ROCK in front of door. +1

Hint 6 of 7:

USE ROCK ON CATAPULT. When guard comes out, MOVE LEVER.  +11

Hint 7 of 7:

Rock lands where guard pauses after walking through door.