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.
You've completed the budget worksheet, what now? Should you find that at the end of the month that you are consistently spending more than you are bringing in, it might be time to take a closer look at where you're spending your money and adjust those areas you can to make up the difference. Should you find, on the other hand, that you consistently have money left over every month, you now have the opportunity to decide what to do with that extra cash. Do you need to build up an emergency or "rainy day" fund? Could you be contributing more to your retirement savings? Could you pay certain loans off faster? Or perhaps you would like to save up for a special or large purchase.