What do I need to do to pay taxes due for 2018 if Federal Return was already filed?

You can use the link below to make a payment online with the IRS (bank debit or credit/debit card) -

https://www.irs.gov/payments

If you prefer to send a physical check, you can use Form 1040V (print and mail with your check) -

https://www.irs.gov/pub/irs-pdf/f1040v.pdf