I completed my 2023 return in Feb 2024, but now need to enter estimated taxes (1040-ES) for 2024. How can I do that in TurboTax?

You cannot use TurboTax to make a 2024 estimated tax payment.

What you can do with the 2023 TurboTax program is create estimated tax forms, Form 1040-ES, for tax year 2024.  Print the forms and mail to the IRS with your payment.

Go to this IRS payment website to make a 2024 estimated tax payment -   https://www.irs.gov/payments

The 2024 tax payments made will need to be entered on your 2024 tax return that is filed next year.

If you want to create the 2024 estimated tax forms, Form1040-ES, then follow this procedure using the online editions and the 2023 return has already been filed -

To access your current or prior year online tax returns sign onto the TurboTax website with the User ID you used to create the account - https://myturbotax.intuit.com/

When you sign onto your online account and land on the Tax Home web page, scroll down and click on Add a state. 

This will take you back to the 2023 online tax return.

Click on Federal>Other Tax Situations>Other Tax Forms>Form W-4 and estimated taxes