[Your Name, & Address here]
[Recipient Name & address here]
As per the court order, your [alimony/child support] of $[amount] falls due on [due date] of every month.
So, your payment for [names or no. of months] is overdue. We request you to please submit payment for this amount within [duration] from the receipt of this letter, if you want to avoid any legal proceedings.
In case you have been facing some problems because of which you were unable to make the payment, please inform us immediately so that we can discuss it and find some solutions.
Look forward to hear from you.
Encl: [Enclosures List here]