To operate the onager you carefully pull the throwing arm back. Then you latch it with the hook and eye. Next you place a projectile in the cup shaped washer. You then need to attach a string to the hook. Grab the string and jerk the hook from the eye hook to fire the catapult. The more tension you put in the torsion bundle (the twisted cord), the farther the catapult will shoot.
Wind the torsion springs. To do this, loop your cord several times through the holes in the upper and lower spring frame and then insert the washers and turning dowels. (You may experiment with different types of cord works best. Nylon and polypropylene work well, but natural fiber twines such as cotton, sisal, and jute may be tried as well.) pull the cord medium tight and then tie off. Place throwing arms in each cord and then twist the looped cord outboard ( so that the throwing arms are pulled towards the outside) until there are many twists above the throwing arm as below. As the cord twists, they will tightly grip the throwing arm (called the braccium by the Greeks) and push it against the head frame. Do this by first twisting the upper dowel and then twisting the bottom dowel, going back and forth between them until they are both tight and there is approximately the same amount of tension on both. When tight, lock the spring into place with the stop peg. Repeat the procedure on the other spring.