The IRS uses Letter 2271C to accept or change your request for an installment payment agreement for your back taxes & it gives instructions for what to do. It tells you the due date of your payments, the amount of the monthly payment, the fee for granting an agreement & other important instructions. After this letter you will start receiving monthly bills (Notice CP521).
Respond to Letter 2271C
It also helps if you can gather evidence of medical, legal, health or other major problem for you, your family or children. This aids in repelling IRS collections as well as making a Penalty Abatement Request.