You would make a payment at www.irs.gov/payments. There will be a similar web site for your state taxes.
The 20% mandatory withholding won't be enough if you are in the 22% or higher tax bracket, and you may owe the additional 10% penalty for early withdrawal depending on your age, so you will probably need to pay additional taxes either at the time of the withdrawal or calculated on your tax return. Remember that no matter what you pay now, it is only an estimate, and the withdrawal must be included on your tax return to calculate the final actual tax owed. If you overpaid your estimate, you will get the difference as a tax refund.
