The 15.3% tax rate is just for the self-employment taxes – the Social Security and Medicare taxes owed on that income. You still need to pay your federal taxes owed on that income too. You can pay your estimated taxes based on the amount you earn each quarter. Q1 is for income earned through 3/31, Q2 is through 5/31, Q3 is through 9/30 and Q4 is for the rest of the year. If you do pay your quarterly estimates based on when the income is earned, make sure to attach a Form 2210 to “annualize your income” and show the income earned per quarter.