You must first file a complaint in the superior court in the county you ex-boyfriend lives in. Go to your local library and review a book called the Rules of Civil Procedure. In that book you will find some forms that you can use as a model for drafting your complaint. However, you might want to hire an attorney to simple prepare the complaint for you, which you will file and represent yourself in court, if that is your intent. If you prepare the complaint yourself, be sure to attach a copy of the promissory note to the complaint; not the original.
Next, contact the clerk of court to determine how much the filing fee is, how many copies you need to file, and if there is a required designation/cover sheet. Comply with all those requirements. Make at least one extra copy of the complaint. The court will time stamp it, and you will retain it for your records.
What happens next really depends on your ex. Once he is served, he will have a date certain by which his response is due to the court. If he fails to respond in a timely manner, you can file a motion for default. Again, refer the Rules of Civil Procedure for a form. If he does answer, you will either have to go to trial, and prove that you are entitled to judgment, or you could file a motion for summary judgment, such a motion is too complex to discuss in this forum. There is also the chance that your ex will consent to an agree judgment entry.
Once, you have your judgment, go back to the clerk’s office and determine if it has proscribed forms for garnishments. If not, check with a local legal supply store, you should be able to find a garnishment form kit. Fill out the forms, and file them with the court in the necessary number of copies, again plus one so you have a copy for your file, and pay the necessary fee. Determine if the court will serve the ex's employer or if you must make service. If it is up to you, send it by certified mail, and file the necessary notice with the court. Payments will be sent from the employer to the clerk of courts, and the clerk will send you payments roughly once a month.
Those are the basics. Good luck.
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