I hope, by replying here, I'll see all the answers you get. I'll need the same next year.
I'm about to do a backdoor to a Roth too. Having just completed my 2020 return, I'm going to do some "What If Scenarios" now. I want to determine the best amount to put into my new Roth.
I just googled "tax form needed for backdoor roth ira", and got what looks to be several good hits. Good luck!
