If you have already filed your tax return, you will mail your 1040-V/payment voucher and your check or money order. You do not need to include your tax return. https://www.irs.gov/pub/irs-pdf/f1040v.pdf
Please see the following instructions for mailing your federal tax payment in with your Form 1040-V:
How To Prepare Your Payment
• Make your check or money order payable to “United States Treasury.” Do not send cash.
• Make sure your name and address appear on your check or money order.
• Enter your daytime phone number and your SSN on your check or money order. If you have an Individual Taxpayer Identification Number (ITIN), enter it wherever your SSN is requested. If you are filing a joint return, enter the SSN shown first on your return. Also enter “2016 Form 1040,” “2016 Form 1040A,” or “2016 Form 1040EZ,” whichever is appropriate.
• To help us process your payment, enter the amount on the right side of your check like this: $ XXX.XX. Do not use dashes or lines (for example, do not enter “$ XXX—” or “$ XXX xx/100”).