How to amend my 2020 return?

Approximately four years ago, I submitted my Form 1040 via TurboTax on time for the 2020 year. Now I want to amend my 2020 return and add $3,000 gift income. Which forms do I need to fill out? Should I do this myself? I think I will owe a little bit of money to the U.S. Treasury, so will I pay with a check from my checking account? Can you tell me what I need to do?  

Thanks.