Interest starts on the shortfall from the day it was due and grows daily. Pay it as soon as you can, not at the next deadline.
Nothing dramatic happens right away. No letter arrives and no fine is issued. The IRS simply starts charging interest on the missed amount from its due date, and the charge compounds until the money comes in. The total is figured quarter by quarter when you file, on Form 2210.
That is why the answer to "when should I pay it" is now, not at the next deadline. Every day of waiting adds interest. You do not need to wait for a quarterly date to send money. A payment made today stops the clock today.
Doubling your next installment does not fix the miss on its own. The catch-up money helps whenever it arrives, but it is credited when paid, so the earlier gap keeps accruing interest until then.
Two things can shrink or erase the damage. If you or your spouse has a W-2 paycheck, extra withholding for the rest of the year is treated as paid evenly across all four quarters, which can wipe out an early miss entirely. And if your income arrived unevenly this year, the annualized income method may show the quarter you "missed" required less than you think.
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Sources: IRS underpayment penalty· IRS Publication 505