If you want to send some in now so you don't owe too much in April on your tax return, you can pay directly on the IRS website https://www.irs.gov/payments
Be sure to pick the right kind of payment and year.....2019 Estimate
When you do your tax return you might get an underpayment Penalty if you owe too much. You might be able to eliminate it or at least reduce it. You can go to Federal Taxes tab or Personal tab, under Other Tax Situations and select Start by the Underpayment Penalties. You will answer a series of questions that may reduce or eliminate the penalty
