Well of course you can't start building a catapult without equipment involved. You must grab enough wood for your catapult and tools; such as screws, hammers, handsaws, rulers, springs, etc. (you get the idea).
You need to know how big you want your catapult and how much wood you might need. You also might want to see how much weight your catapult might carry, if it's too big, you might want to get some assistance. This is where you use a ruler and begin to measure it. You must need a flat base that at least touches the ground. The measurements must be exact so no mistakes will interfer when you're putting it together.