If you're self_employed, your after_tax income equals your gross income less your business expenses, such as the cost of your laptop or airfare to conferences, as well as the amount you set aside for taxes. You're responsible for remitting your own quarterly estimated tax payments to the government because you don't have an employer to take care of it for you.
Begin by listing your income for the month. This should include any paychecks you receive, as well as income from other sources like child support or investments. If you have a business, you should include the amount that you pay yourself out of the business each month.