Determine whether you are mailing your payment as an estimated installment payment or your are mailing your payment with your annual tax return. Different forms accompany these payments.
Consult the IRS website or your 1040-ES form to find the appropriate address. Different U.S. regions have different mailing addresses. For example, most western states residents send estimated payments to the processing center in San Francisco, CA; while residents in southeastern states send estimated payments to the processing center in Atlanta, GA.
Mail the check and the appropriate documentation to the appropriate mailing address. You can also do this online at the IRS.gov website.