IRS Form 9465, Request for Installment Payments will be needed to start the process of requesting an installment agreement for your back taxes.
You can obtain this form by calling the IRS at 1-800-829-1040. Or, you can go online to their website and order it. If it is easier for you, once you are visiting the IRS website, you can easily just print out the form.
Even though it is not required that you prepare and submit a Form 9465, it is recommended.