I sold stock at a profit in January 2018. How to I pay estimated taxes to avoid penalties?

[TomYoung]

Do you know that you'll have penalties if you don't make estimated tax payments, or are you just guessing?  Being "underpaid" doesn't simply mean that you owe the IRS a lot of money when you submit your income tax return; you can owe a ton of money and still not be underpaid, and incur no penalties.

I'd suggest working through the "W-4 and estimated taxes" interview to see if the regular tax withholding from your paycheck gets you into one of the "safe harbors" to avoid the underpayment penalty and not make an interest-free loan to the government.  As a bonus that interview will calculate optional amounts for estimated taxes - there's different "safe harbors" you can get into - and prepare the Form 1040ES vouchers for you.

[Opus 17]

You can make a payment at www.irs.gov/payments.  Be sure to select "2018 form 1040-ES" as the reason.  The long term capital gains rate is 15% for most taxpayers of the gain (difference between selling price and purchase price).