A lot depends on when she received $$ for that income, how much y'all owed last year on your MFJ tax return, and how much you will owe this year. The details on that can get messy.
But simply put, at this point you should probably make an estimated tax payment of 20-25% of her net gain (after allowed business expenses) before 15 Jan 2020. That may eliminate, or reduce any penalty for late payment of expected taxes on that income.....but again, that depends on the details of when she made those $$ during the year.
In the future, she needs to pay estimated taxes quarterly during the year according to how much she makes each quarter. IF you want to do so, instead of doing that you could also increase your withholding at work to cover her expected required tax payments instead....since you will file MFJ, it's all figured in together. BUT..if for some reason you plan to file MFS (don't...only very rarely should a couple file Federal taxes separately) then she would need to make those payments separately by 1040ES.
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1040 ES direct Debit payments can be made on the IRS website (You'll have to check whatever state you live in for their payment procedures):
https://www.irs.gov/payments/direct-pay
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Or, there are mail-in forms if you want to send in a check, but you'd only use the 4th Qtr form:
https://www.irs.gov/pub/irs-pdf/f1040es.pdf
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
