If I received the first stimulus check, but not the second, where do I notate that when filing my taxes?

There will be a screen with 2 boxes for each round.  If you didn't get one enter a 0 in the box.

When you go through the federal review tab it will ask how much you got if any.  Or to enter your stimulus payment in TurboTax, type "stimulus" (without the quotes) in the Search box, then click the link that says "Jump to stimulus."

After you have entered your tax information and are ready for the Federal Review, it will be the first question.

If you are trying to trigger it, you can go to Other Situations, answer any questions and click Done. The next automatic selection is Federal Review and it will ask about payments received.

The Recovery rebate credit is on 1040 line 30.  It either adds to your refund or reduced a tax due.

Before filing,  You can preview the 1040 or print the whole return

https://ttlc.intuit.com/community/accessing/help/how-do-i-preview-my-turbotax-online-return-before-filing/00/26160