Turning lag bolts in 4x4's is pretty annoying, isn't it? Drill two huge (but not too big. Keep turning until the little pointy part sticks out of the other end of the wood, then re-clamp it and finish up. You can take it down from the posts now if it makes it any easier.
Now, start screwing your lag bolts into the cross-brace piece. The cross-brace's top should be flush with the top of the two posts, and should be closer to the shorter side of the base of the frame. To install a cross-brace, first pre-drill four holes in that leftover 3' 4x4 that you cut earlier (two on each side), after clamping it to the top of the two 4x4 posts we put up in the last step. Otherwise, follow the instructions below. If you don't want a cross-brace, you'll have to get two more u-bolts and bolt them to the top of the posts so that they face eachother. If you want to go true catapult, the cross-brace is essential, but I highly advise against it.
Later on, we found that this really didn't work, and ended breaking a really nice fiberglass pole because of it (and the insane momentum it had). So we originally planned to have a cross-brace, which would stop the catapult arm from aiming down and force the release of the projectile at a 90 degree angle to the ground.
