You will not owe any taxes, you are just reporting that you made a non-deductible IRA for 2018. Put that in the explanation of the amended return. You include the 1040x and the 8606 - https://www.irs.gov/faqs/irs-procedures/amended-returns-form-1040x
You should not have to amend 2019 as you are going to report it correctly for this year, showing the prior year basis and the contribution for 2019 even though you made it in 2020.
