It might not be too bad, because there are rarely penalties if you paid/withheld at least the amount owed the previous year.

Depending on the size of the gig, it is worth it to even pay one payment to estimated taxes. There is no law that says you have to submit all 4 forms, but again I am NOT a tax professional. I simply know that in past years when I had a salary and sidework, I paid only one or two estimated tax payments in the quarter I earned the cash.

If this is significant work, consider running the numbers and paying something. For federal taxes, estimate 20%. If that number is big to you and would be hard to pay back next April, make the payments.

Don’t forget that if you have state income tax you need to make state income tax payments.

One shortcut? Simply ask the nice folks at your workplace to deduct $X extra each pay for taxes and/or claim “0” dependents so the maximum is taken out, if that would cover your tax liability, including beyond the job itself.