Calculate SE tax, quarterly estimated payments, and your true net income.
As a self-employed person you pay both the employee and employer portions of FICA taxes: 12.4% Social Security + 2.9% Medicare = 15.3% total SE tax, applied to 92.35% of net earnings (to account for the employer deduction). You can then deduct half of SE tax from your adjusted gross income, reducing your federal income tax.
Quarterly estimated taxes are due four times a year. Missing them can trigger IRS penalties even if you pay in full at year end.