Go to the family court in your county and speak with the clerk's office. They will provide you with a violation petition to fill out. File that with the clerk's office and your case will be put on the court's calendar.
The above answer is intended for informational purposes and is not legal advice. It does not constitute the creation of an attorney-client relationship.
File a violation petition with the Family Court. You will then need to arrange to have him served with the petition. You're best advised to do so via a process server or the Sheriff's office. Schedule a consult with a NYC Divorce lawyer for a full assessment.
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File the violation petition in family court. As others have suggested, it is really quite simple. Make sure to bring the order/agreement when you go to file. Going forward request the assistance of support collection.
The content of this post is not legal advice and should not be relied upon as such. I am not your attorney and no attorney-client relationship has been formed based upon this exchange.