WAIT WAIT WAIT ....
The IRS will stop mailing checks on 1/15 so if you don't get it by the time the efiling opens at the end of the month you can apply for the missing money on the 2020 return.
CAUTION to those eager to jump the gun ... if you do NOT enter the correct amount of stimulus the IRS did pay you in advance then expect your eagerness to delay your return being processed. And since the IRS is still suffering from understaffing due to the covid situation and perpetual underfunding and they are also stretched thin managing the last minute tax law/form changes on top of sending out second stimulus payments expect the delays to be long.
The questions are in the section Federal Review, near the end of the TurboTax interview.
Once the information is entered you will see your refund increase or balance due decrease, and TurboTax will enter the amount of the credit on Line 30 of Form 1040 for you.