You can write this easily. Just RE: the style of the case, address it to him with your request and give him 10 days from the date of your letter to respond. (certified return receipt if you are doing this yourself). After time has passed and he still hasn't paid, you will need to proceed without hesitation. It will be quicker and easier to hire an atty. to prep, file and serve this for you so that there are no errors on your behalf. We are reasonably priced should you need our services.
Don’t worry about getting bogged down in legalese. You can write that letter in natural language just so long as you touch on each of the requirements. The danger of asking for a pre-drafted letter is that you might end up with something you don’t understand that has effects you didn’t intend.
One thing to consider about hiring an attorney on a contempt action is the possibility for seeking attorney’s fees. The burden of proof is on the party seeking attorney’s fees and is dependant on the other party being found in contempt, but, an qualified family law attorney could advise you of the likelihood of securing attorney’s fees.
*** This is a general interest posting on a public website, regarding general legal concepts. This posting is not, and should not be interpreted to be legal advice to anyone, or to create an attorney client relationship between the poster and anyone. There is no way the amount of information in a website posting could be sufficient for the poster to provide informed legal advice. If you have legal questions pertaining to your situation, you should discuss them with an attorney, in person, and in sufficient detail for that attorney to provide you with confidential, competent legal advice ***
I agree with the other contributors - just based off your note here, you can write the letter to your ex. As for it being "certified," that probably is referring to how you mail the letter from the Post Office. Certified, return receipt requested. Good luck, and consider mediation and arbitration in these matters.