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The first step is to find an attorney in your State to write a letter to your ex asking him for payment plan for the funds.
The second step is to go back to Family Court where the agreement was issued in order to have the judge enforce the agreement and bring your ex back to court to make him pay.
If that does not work, you can try to sue him in small claims court but the amounts that you are able to get there are less and they may still send you back to Family Court.