If you are talking about the balance due on your return, you can either:
- Send in a check for the entire balance due with the payment voucher (1040-V) from your return. Make sure you are not confusing the balance due on the 2020 return with the four estimated payments for 2021 (using 1040-ES) ; or
- Make the payment online at the IRS website: https://www.irs.gov/payments
