I took a covid related retirement withdrawal and paid taxes on 1/3 of that amount in 2020. Where and how do I enter the next 1/3 in 2021 taxes?

I cannot figure out how to do this either... I'm not sure how else to follow this thread, so I'm replying. 

The ability to enter the payment has not yet been implemented in TurboTax. The IRS has not even released the form for it yet.  Once those forms are implemented you will visit the "Retirement" topic in the "Wages and Income" tab.  If you have Forms 1099-R for TY2021 distributions, you can enter them as usual. 

If not, you just select "NO" when asked if you received a 1099-R. 

At either point, you will then be asked about prior year "Disaster Distributions."  Follow the prompts from there to enter the second installment of your distribution.