Ok ... first amend the 2020 return to add the non deductible IRA contributions which will produce a form 8606 reporting the basis.
Then convert the 2020 contribution amount to the ROTH in 2021. The non deductible portion of the 2020 IRA will show on the 2021 form 8606 and if the year end value of all your traditional IRAs is zero then only the earnings converted will be taxable so do the conversion asap.
