Since you don't know how much income you'll make each year, you may wish to fill out your W4s without the business information and then set aside funds to make quarterly estimated payments.
You can then base each estimate on the amount of money that you've made that quarter. This would require making sure that your bookkeeping is in good order by the early part of each quarter, as the payments are due on the 15th day after quarter end. Most bookkeeping programs, like QuickBooks, can help you determine the amount to pay for the estimated taxes.
You can use blank Form 1040-V to send in those payments, or make them online.
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