How does a separation agreement get enforced?

It depends what you want enforced. Generally, you would file a motion to compel, though that is often a waste of time and you might as well file for contempt of court for failing to follow the agreement. If it is something like signing a document, the clerk of the court often can sign for the other spouse if they are refusing to cooperate. If you choose to hire a lawyer to help, courts often grant attorney fees for having to take action to enforce the agreement. I hope this helps.