This document is a purchase agreement between a seller and buyer. A payment plan agreement outlines an installment plan to repay an outstanding balance over a specified time frame.How would I fill out the bottom of a payment agreement when it says signed this ____ day of ______, ____, at ______, ______? How to fill out the Installment Purchase Agreement Template for Users? 1. Read through the agreement thoroughly. 2. The Buyer agrees to pay. I'm gonna break down all the terms and conditions so you can understand what it means and we're gonna fill that out so you can have a real-life example of it. A payment plan agreement is a legal document outlining how a borrower agrees to pay back a lender over time.