Yes, you can go to www.irs.gov/payments. You would select 2025 as the tax year and estimated taxes as the reason. You would get a receipt, keep that for when you file, but you don't need other paperwork.
For a conversion done in April, and with no withholding, the IRS will want to see payment in full by June 15, and you will have to use the annualized method of calculating the penalty on your tax return. Or, you can pay 1/4 of the taxes on or before April 15, June 15, Sept 15, and Jan 15, 2026, and you would not have to use the annualized method. (Because the IRS wants tax payments spread out evenly over the year.). So you can actually make slower payments and keep your money longer if you make one early payment (before April 15, even if the conversion settles after April 15), then you can make 3 more payments spread out.
