You need to pay in advance one way or another. You can either make an estimated tax payment, or increase your withholding for the rest of the year to cover the tax on the capital gain.
If you want to make an estimated tax payment you don't have to do the whole worksheet. The worksheet is just to help you come up with an amount to pay. You can just send a 1040-ES voucher with a check for whatever amount you think is appropriate, or you can make the payment online from your bank account using Direct Pay on the IRS web site. You don't have to make any additional quarterly payments.
If you want to try adjusting your withholding you could use the Tax Withholding Estimator on the IRS web site. However, the Tax Withholding Estimator does not take into account the lower tax rates on long-term capital gains, so if your gain is long-term you would have to fudge the amount of income to compensate for the lower rate.
