The thing you need to do is get a handle on what your tax liability will be before April 15th. To do this we recommend using a Tax Calculator: https://turbotax.intuit.com/tax-tools/calculators/taxcaster/.
Use your paystubs and best estimate for your gig work (this would go under Self-employment). After you do the calculation you can make an estimated payment to make up any tax that may be due. The gig work makes it difficult to use a W-4 to cover your taxes because of its unpredictability.
