On the 2021 return you would have filed Form 8606 reporting the $6,000 TY2021 Non-deductible IRA contribution, establishing a contribution basis of $6,000.

On the 2022 return you report another $6,000 TY2022 Non-deductible IRA on Form 8606, bringing your total contribution basis to $12,000.

On the 2022 return you'll indicate the FMV of IRAs on 12/31/22 of $0.  That should allocate all of the basis to the $12,000 conversion amount on the 1099-R.

If your question is more along the lines of "how do I enter this in the software", that might be a question for @George4Tacks