The basic scheme for this experiment is we take the catapult and set it in a large classroom. The swing arm is intercepted by two photogates that are placed at some distance apart, near the end of the swing arms travel. These photogates are in turn attached to a timer which reads some time for the swing arm to travel. The arm is released with some payload and a time is received on the timer. We use this time to calculate a final velocity and run through the formulas on the Physics of the Project page and calculate our desired results.
As it turns out, our max range is the easiest data to calculate, and it is fairly accurate, since you can imagine there are other factors at play, not only simple kinematics formulas can determine. More information can be found in my lesson plan on actual lesson topics