as a possibility before you decide to mail in your return, try using 0 for the AGI. sometimes that works. sometimes not. if you can't e-file
follow TT filing instructions. The amount will be specified. you can mail a check with the return or use the IRS website to pay. https://www.irs.gov/payments
if you have a state return follow TT filing instructions for the state. Most states allow payment through its website.
best would be to use a filing method where you get proof of delivery.
by the way, the balance due was supposed to be paid by 7/15/2020 so you might get a notice from the IRS for late payment penalties and interest.
