Last year, I applied part of my refund to estimated tax for the next year (2018). Where in TurboTax to I apply that overpayment to my return?

To enter, change or delete a refund from the prior year applied to this years taxes (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 options under 2017 Refund Applied to 2018

Or enter refund applied to this year in the Search box located in the upper right of the program screen.  Click on Jump to refund applied to this year

Thank you. This was extremely helpful.

If it didn't transfer over from last year then enter a refund from last year that you applied to this year here.   BUT first make sure it's not already there.  You don't want to apply it twice.
Federal 
Deductions and Credits at top
Then scroll way down to Estimates and Other Taxes Paid
Other Income Taxes - Click the Start or Revisit

Next page scroll down to 2017 Refund Applied to 2018
Click Start or Update by the tax you applied