For the 2020 tax return you would have filed a Form 8606 showing the $6K non-deductible contribution on line 1.

For the 2021 tax return you would have filed a Form 8606 showing another $6K non-deductible contribution on line 1 and the first $6K of basis on line 2.  Now the total basis in the IRA is $12K (line 3).  I don't know what software you're using but you may need to enter $0 as the FMV of Traditional IRAs on 12/31/21 to get the rest of Part I to populate and carry $0 to the 1040 line 4b.

This assumes there are no other non-Roth IRAs (including SEP-IRAs, SIMPLE-IRAs, etc.)

Rick