How do I enforce a contract if the client breaks it?

I see a lot of people talking about the importance of having a contract. I'm just wondering how to enforce the contract if the client breaks it?

So the contract does two things.

Firstly, it gives you a legal basis to enforce breaches.

You set out what you are going to deliver and the payment terms and then you both agree to it. You deliver as promised and the client doesn't pay. That's a breach of contract, everything's laid out unambiguously on the piece of paper and it's got both of your s…