Agreements
When someone agrees to pay, put it in writing before the first installment. These documents fix the amount, the schedule, and what happens on a missed payment, and both parties sign.
Each page includes a complete sample you can read first. The builder fills it with your terms for $19.99, with PDF and Word downloads.
Coming from a demand letter?
That is the natural path: the demand letter gets them to agree, and the agreement here makes it stick. If you have not sent the letter yet, start there.