I would like to make additional contributions to my 2019 retirement accounts (Roth, 401K, etc.) for tax purposes in 2020. Can someone help me with how to do that?

You cannot make additional contributions to a 401k for 2019 tax purposes at this point of the year, unless it is a self-employed 401k.

 

You can make a contribution to a Roth IRA for 2019 if you are eligible, but there would probably be no tax advantage on your return for doing so.  The contribution to a Roth IRA is not deductible.  The only tax advantage might be the Saver's Credit if your income were low enough for you to qualify for that credit.  

 

Making a Traditional IRA contribution if eligible or increasing your Health Savings Account (HSA) contribution can create an additional deduction on your return and both of those can be done through the due date of your return.  However, there are income limits and other restrictions.