When I am preparing my estimated taxes, should I be entering my year-to-date withholding or my estimated withholding for the full year?

If you are preparing quarterly estimates, this is to estimate how much you need to pay that isn't covered by your withholding, and how much you need to pay quarterly in order not to have an underpayment penalty. You can do this in TurboTax - just answer the questions in the program, which will tell you whether it means for the year or year-to-date. You can change your W-4 form that you file with an employer as well. Since it's already April, knowing how much you've paid in so far this year can help in figuring how much more you need to add to your current payment plan for the rest of the year if applicable.

When you prepare your 2024 return, TurboTax will automatically calculate your 2025 estimated tax payments and prepare 1040-ES vouchers if we think you may be at risk for an underpayment penalty next year.

You can also calculate your 2025 estimated taxes and generate payment vouchers by doing this:

1. Open or continue your return.
2. Navigate to the 1040-ES section:
   • TurboTax Online/Mobile: Go to 1040-ES.
   • TurboTax Desktop: Search for 1040-es (be sure to include the dash) and select the Jump to link.
3. Answer No to the question Do you want to change your W-4 withholdings for 2025?
4. Answer questions about your 2025 filing status, income, and deductions.
5. Eventually, you'll come to the Print Vouchers? screen. Answer Yes and we'll include your 2025 1040-ES payment vouchers when you print a copy of your return later.

See this help article and this one for more information.

This TurboTax tips article includes a good explanation of quarterly estimates.