There is something called a lease contract or a lease agreement. It contains all the details, stipulations, terms and conditions for the contract which usually will be set by the retailer.
Along with this, your details will also be mentioned in the contract. The amount of payment to be paid in totality, along with the downpayment and monthly installments will be specified.
The term of installment will also be mentioned, and a fine for late payment, if any can be there in the agreement. This contract is a binding document in the court of law and any breach therein by you or the retailer can be questioned In the court of law.
The exact procedures to be followed are going to be different from retailer to retailer. You need to make sure you know all the terms and conditions before signing this contract.
Once you sign this contract, you will be bound by it, and it will remain valid until the last installment.