I have an existing IRS installment agreement, but can only make a partial payment for what I owe for 2018. How should I pay the tax I owe this year?
If you owe taxes and make a partial payment, the IRS will charge interest on any unpaid balance. This applies to a balance not postmarked by April 15, 2019.
You can arrange another installment agreement to cover the balance for this year. If you do, contact the IRS to have the filing agreements consolidated.