RE: Well, except that the $6000 for tax year 2020 should be on the 2020 tax return.

Just wanted to clarify as follows:

In @TaxGuyBill 's scenario, $6K should be reported as non-deductible IRA on the 2020 return and $6K on the 2021 return.  The basis upon conversion from the said contributions would be $12K on the 2021 return. 

If the $6K was NOT reported as non-deductible IRA on the 2020 return, some would suggest to amend it, but that would be a different consideration. 

In this scenario, the BEGINNING basis of the IRA for 2021 should include the $6K.  (Check if there had been other non-deductible IRA contributions made in prior years.)  Upon the conversion, the basis should be $12K.