@mytax26 for simplicity, assume that the capital gains tax is 15%. Just make that estimated payment now.
What is your income (including your capital gains) and your filing status. Depending on your response, I may tweak my simple answer.
Federal payments can be made at this website:
https://www.irs.gov/payments/direct-pay-with-bank-account
When you complete your tax return, TT will ask you if you made estimated payments, so you should be all set there.
Yes, there is a more complicated way to figure out if you are subject to underpayment penalties, but this is the simple approach.
your question was how to calculate the tax and make the estimated payment. The question was not how to calculate whether or not you are subject to underpayment penaties :-).
