To fill out a money order you need to first fill out the who the payee is, the purchasers information and sign it. Write your address in the purchaser section.Include your account number if you're paying a bill. I'm going to show you how to fill out a money order from the USPS or the United States Postal Service. Please refer to the annotated image and instructions below to learn how to fill out a check for your OPT application. Sign the money order where it states – "Signature," "Purchaser," or "Drawer. " Do not sign the back of the document, that is for the recipient to sign. Decide on the money order amount. 1. Buy a money order for the desired amount. 2. Fill in the "Pay to the Order Of" line. 3.