Lubricate the bolts really well with grease. Make sure they thread in without resistance; if there is resistance even with grease, you might need to chase the threads with a tap and/or get a new bolt. Tighten slowly and evenly until the bars don't move when you jump on them. The key is to tighten evenly, which is really tough to do with 4 bolt stems, especially with a clamp that is an order of magnitude more stiff than the bars you are clamping.

This is another way of saying there is a learning curve, and there is no way to shortcut that. If you are new to tightening bolts over fragile joints, take it extremely slow else you end up like the OP. Also, own your mistakes and learn from them. If you can't afford to make mistakes, let a shop do the work or work with more robust parts.