backdoor IRA contribution for year 2019 and IRA conversion

I know there have been many backdoor IRA questions but my situation is bit complex, so any answer would be greatly appreciated!

My timeline is

1. On july 2019, I rolled over my previous employer pre tax 401K to vanguard Traditional IRA (T IRA)

2. On Jan 2020, after learning about backdoor IRA, I rolled over my entire T ira to my current 401k plan , thus made the Tira balance to 0

3. On Feb 2020, I made a contribution of after tax amount of $12000  from my bank account to my T ira account, $6000 for 2019, and $6000 for year 2020

4. After one day,  I converted entire $12000 to my Roth IRA

 

Considering my Traditional Ira balance was not 0 on 12/31/2019, and I made contribution of $6000 for year 2019 on 02/2020. can someone kindly show me the steps to fill in the form for 2019 tax return? any possibly 2020 tax return? thank you so much!