You can enter estimated taxes you paid by following these steps:

• In TurboTax, open your tax return
• Click on Federal in the left-hand column, then Deductions & Credits on top of the screen
• Scroll down to All tax breaks, locate the section named Estimates and Other Taxes Paid and click Show more
• Click on Start next to Estimated Tax Payments
• On the next page, click Start next to Federal estimated taxes for 2018 (form 1040ES) (see attached screenshot)

Thank you for this answer, but there is not an "Estimates and Other Taxes Paid" section in my version. Only a "Income Taxes Paid," which only offers tax payments for extensions. What am I missing?

Which TurboTax version are you using?
If you are using TurboTax Self-Employed, please follow exactly the instructions above. Scroll down the screen and you should see a section called Estimates and Other Taxes Paid. Click Show more next to it and you will see Federal estimated taxes for 2018. I have just checked on my version of TurboTax Self-Employed.