How do I account for a payment already made to the IRS

What type of payment are you referring to? 

Payment of federal taxes owed for a prior tax year are not entered on a federal tax return.

To enter, edit or delete estimated taxes paid (Federal, State, Local) -

• Click on Federal Taxes (Personal using Home and Business)
• Click on Deductions and Credits
• Click on I'll choose what I work on (if shown)
• Scroll down to Estimates and Other Taxes Paid
• On Estimates, click on the start or update button

Or enter estimates paid in the Search box located in the upper right of the online program screen. Click on Jump to estimates paid

Federal estimated taxes paid are entered on Form 1040 Schedule 3 Line 8. The totals for Schedule 3 flow to Form 1040 Line 18d 

To enter, change or delete a payment made with an extension request (Federal, State, Local) -

• Click on Federal Taxes (Personal using Home and Business)
• Click on Deductions and Credits
• Click on I'll choose what I work on (if shown)
• Scroll down to Estimates and Other Taxes Paid
• On Other Income Taxes, click on the start or update button

On the next screen select the type of extension payment made and click on the start or update button

Or enter federal extension payment in the Search box located in the upper right of the program screen. Click on Jump to federal extension payment

The federal extension payment will be shown on Schedule 3 Line 10. The amount from Schedule 3 Part II Line 14 flows to Form 1040 Line 18d