Basically you just make a fourth-quarter estimated tax payment to cover the tax on the income from the big project. A few hundred dollars of additional income earlier in the year is not going to result in any huge penalties. There might not be any penalty at all. But if you're worried about it, you could add a little extra to your fourth quarter estimated tax payment to cover the tax on the earlier income. That would reduce any possible penalty, but not necessarily completely eliminate any penalty.
When you prepare your tax return for 2019, if it initially shows an underpayment penalty, using the annualized method will probably reduce the penalty. The annualized method takes account of the fact that the majority of the freelance income was received late in the year. TurboTax will lead you through it.
Here are two other possible ways to "go about this."
1. If you have a job where you get a W-2, have additional tax withheld from your pay for the rest of the year. If possible, have enough withheld to cover all the tax on your freelance income, instead of making an estimated tax payment. In calculating any penalty, the withholding is treated as if it was paid evenly during the year, even though more was withheld in November and December. That reduces any possible penalty, and can eliminate the need to use the annualized method. If you do this, be sure to change the withholding again in January to put it back to the way it was. Discuss this with your payroll department so that they know what you're doing and will handle it properly.
2. If possible, don't bill for the big freelance project until the very end of December, or in January. The income is taxed in the year that you receive it, regardless of when you did the work. So if you can defer the income to next year, you defer the tax. You can then make a first-quarter 2020 estimated tax payment to cover it, or adjust the 2020 withholding for your W-2 job. If you want to do this, discuss it with the client to make sure it doesn't cause any problems for them.
