The address to mail your payment will be on your Form 1040-V, or you can get it off the  IRS 1040-V instructions  (page 2).

If you owe additional taxes when you file your return, select the option  I will mail a check  when asked how you want to pay. This question will come up when you go through the  File  section.

• Your check or money order should be made out to United States Treasury.
• Make sure these 4 items are on the front the check or money order:
  1. Social Security number
  2. Taxpayer's name
  3. Your mailing address
  4. Daytime phone number
• Mail the check or money order to the IRS, along with Form 1040-V, postmarked by the April 17, 2018 tax filing deadline to avoid the late payment penalty.

Tip:  Even if you selected the check payment option, you can always  make an electronic payment to the IRS  instead of sending a check.