See form 2210 for the safe harbor calculation, also form 1040-ES. To avoid penalty you need to have paid through timely (usually quarterly) estimated tax (ES) or withholding, the smaller of 100% of your 2024 tax (110% if AGI > 150k or 75k if filing MFS), to 90% of your 2025 tax. Withholding is always considered "timely" regardless of when it was taken vs. your income.
If your income/tax increased in 2025 vs. 2024 due to the cap gains it may be that your optimal safe harbor would be based on 2024 tax and if you met that thru withholding then you don't need to pay ES (but still owe balance of 2025 tax when you file in April).
To determine your 2025 tax you can try an online calculator (TT 2025 online won't be open til Dec, and the 2025 desktop s/w doesn't have 1099B section yet) or consult 1040-ES or IRS withholding calculator may help. Long term cap gain rate depends on your income whether it's 0,15 or 20%. Short term cap gains are taxed at ordinary tax rates.
If you do end up needing to pay ES it's late in the year to do it, you are probably going to have a penalty unless you covered it via withholding already; but if you pay what was due for Q1-3 ASAP (direct at irs.gov, avoid checks and vouchers), you can stop the penalty accruing further, then pay the Q4 installment by January. If your income/gains is not even thru the year and occurs in later quarters you may be able to reduce (but not eliminate) the penalty by filing Form 2210 Annualized Income (AI) method where you have to break down your AGI/withholding/cap gains etc by quarter (not even - 3/31, 5/31, 8/31, 12/31), it can be some extra calculation/filing work. You can work thru this option and choose whether to adopt it, when you file under Other Tax Situations / Underpayment Penalties (otherwise TT will calculate the IRS default penalty assuming even income over the year). When you prep your return, you enter any ES paid in the Deductions & Credits section, Estimates and Other Taxes Paid.
Likewise for next year apply the same process to avoid a penalty in 2026. If you expect similar cap gains next year the easiest way is to pay 2026 ES based on 2025 tax which will be known by April (2026 withholding still needs to be estimated), if you pay that amount quarterly on time then for penalty purposes it doesn't matter the timing or amount of income. In Turbotax you can work through options under Other Tax Situations / Form W4 and Estimated Taxes.
Don't forget state also if applicable, they will likely have similar estimated tax requirements.
Hope that helps get started - consult Form 2210 lines 1-9 for the safe harbor calcs.
