As stated in the instructions for the Form 433-D, Installment Agreement - https://www.irs.gov/pub/irs-pdf/f433d.pdf

Review the terms of this agreement. When you’ve completed this agreement form, please sign and date it. Then, return Part 1 to IRS at the address on the letter that came with it or the address shown in the “For assistance” box on the front of the form.

According to the instructions:   Review the terms of this agreement.
When you’ve completed this agreement form, please sign and date it. Then, return Part 1 to IRS at the address on the letter
that came with it or the address shown in the “For assistance” box on the front of the form.

https://www.irs.gov/pub/irs-access/f433d_accessible.pdf

Or if you received an IRS CP522 Notice - https://www.irs.gov/individuals/understanding-your-cp522-notice

What you need to do

• Complete the enclosed Form 433-F, Collection Information StatementPDF, with your current income and expenses.
• Call us at 800-831-0273 with the information from Form 433-DPDF. Do not mail the form to us. You must call us within 10 days from the date of your notice. Otherwise, your installment agreement will default and we’ll bill you for the full amount of tax you owe.