How can I fill out "How much did you pay in federal estimated taxes?" section? Which box of W-2 should I check to fill out the section?

[PaulaM]

If you had taxes withheld from your W-2, then you probably did not make estimated taxes and don't need to enter anything in that section. All you tax withholdings were entered when you entered your W-2.

[DoninGA]

Your W-2 does not have estimated taxes.  It only has the federal income taxes that were withheld from your wages.

Estimated taxes are those taxes that you paid directly to the IRS or your state by either completing and mailing an Estimated Tax Payment form with a check or using the IRS or State website to have your estimated tax payment directly debited from your bank account.

If you did not do any of the above then you did not pay any estimated taxes.