You should have completed the 8606 on your 2018 return, it would then have your "basis" set at $5,500. You can file an amended return to report that.
Your 8606 for 2019 would add to that basis the 2019 contribution of $6,000. You then converted $5,500 in 2019 and report that on the Roth conversion section. This will reduce the basis you will have for the 2020 8606 (which if you get back on making the contribution in the same year, will then use all your basis next year when you report the conversions of both the 2019 and the 2020 contributions.)
