You still do your taxes in April and if you make no payments during the year you'd pay income tax and Social Security and Medicare taxes all then.

Whether you should be making payments during the year depends on how much you earn, your tax last year, and what you expect to have as tax this year. A booklet on this topic is called Form 1040-ES, because the 4 periodic payments are called "EStimated taxes."

With your W-2 jobs in past you are right that taxes were taken out in advance for you.

Instead, you calculate taxes using:

Schedule C-EZ or C: revenue minus deductible expenses equals profit.

Schedule SE: the Soc Sec and Medicare taxes will be profit * 92.35% * 15.3%.

Form 1040 line 12 is where Schedule C profit is listed, then put 1/2 SE taxes on line 27 as a deduction and put SE taxes on line 57. You work through to find income tax after you subtract standard deduction suitable for your filing status, and personal exemption if you are not a dependent.

You may want to mock up taxes now or in early January in case making an estimated tax payment is advisable. If you wait until mid-Apr and owe more than $1000, there could be a small penalty for not making payments more promptly.