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@1548432s the problem at this point with increasing withholding is it's end Oct so by the time your payroll processes a withholding change it probably won't help much for 2025.  But hopefully you don’t owe much Estimated Tax (ES) or penalty if your 2024 tax was much lower before the 1099 gig.

You should go thru the safe harbor calculation shown by @guywong and if this was a new 1099 gig in 2025 and boost to income compared to 2024, then you may have already covered the ES due based on meeting 100/110% of 2024 tax thru withholding; if so, then for ES and penalty purposes it doesn't matter how much extra you earned in 2025 or when.  The rest of the tax is owed in April but at least you won't have to pay a large amount now, so that can earn interest until April.

If there is some amount to make up to meet the safe harbor amount that you can't do via withholding, then as others have said you should pay the overdue ES on this ASAP to stop the penalty accruing further.  To minimize the default penalty calc, pay 75% of the total ES due for the year now (for Q1-3), and the remaining quarterly amount by 1/15 **.  Since the 1099 gig started later in the year you may also be able to file 2210 Annualized Income method to reduce (but likely not eliminate) the penalty, for this you have to break down your AGI/withholding on a quarterly basis which can be extra calcs and filing work for you, whether that's worthwhile depends the penalty outcome (and your state may have similar).  IRS doesn't know your gig started in July so will assume ES was due on it from Q1 and that's the default penalty calculation that Turbotax will make; the AI method will shift the income out into the later quarters which will reduce the penalty.

** @Opus 17 commented above "At this point you should make your Sept 15 estimated payment now, for income earned from June 1-Aug 31, and plan to make a payment by Jan 15 for income earned from Sept 1-Dec 31" - which is likely different to 75%/25% I suggest above.  I think, it depends the outcome of the safe harbor calculation, whether you are using prior year or current year tax and how much ES is due.  Paying a large amount of ES unevenly based on specific income periods would certainly need the Form 2210 AI method.  Otherwise by default you need to have paid by now at least 75% of the ES or will continue to accrue penalty (and can always decide to adopt the AI method later if it helps).  But @Opus 17 pls do correct me on anything.

Refer to Form 2210 lines 1-9 for the safe harbor calculation and other parts of 2210 instructions for info on penalty calcs and AI method.  Form 1040-ES may also be helpful.

When you file with Turbotax you can review the penalty situation in Other Tax Situations / Underpayment Penalty section and work through the AI method there.

Looking ahead for 2026 tax year - if the 1099 gig is going to continue and you can't cover the expected ES with W2 withholding, then you should set up quarterly ES payments; go through the safe harbor calculation for 2026 when go through your 2025 filing so the 'prior year' tax for 2025 is known.  You can work through estimates for this after you file in Other Tax Situations / Form W4 and Estimated Taxes (or other tools like the IRS Tax Withholding Estimator).

As others have said you should pay directly at irs.gov, avoid checks and vouchers.  When you make a payment be sure to classify it for the correct purpose (ES vs. final tax due etc) and tax year.  I also prefer to make my final return payment directly at irs.gov rather than submit payment details in Turbotax etc, select the option to pay by mail/check etc to bypass payment details, then just pay directly at irs.gov by the April deadline.  IRS will debit your account quickly, within the next business day, so have the money lined up when you submit it on the website.

Anyway - start with the safe harbor calc to see where you really stand on ES, and if you have questions once that is known folks here will be able to advise further.