"Self Employment tax" (SET) is social security and medicare tax (known as FICA on your W-2 pay stub). SET is how the self employed pay their FICA.
If you already pay the maximum social security tax on your w-2 wages (wages over $132,900), then your SET would only be 2.9%.
TurboTax (TT) can prepare the quarterly payment vouchers. In your 2018 software, enter at:
Federal Taxes
-Other Tax Situations
-Other Tax Forms
-Form W-4 and Estimated Taxes - Click the Start or Update button
On the next screen answer No to the W-4 question
If your goal is just to avoid the underpayment penalty, then paying 100% of your prior year tax liability is the “safe haven”
Or you can obtain blank IRS Form 1040-ES from the IRS. The form and instructions are at this link: https://www.irs.gov/pub/irs-pdf/f1040es.pdf
You can also pay Federal directly here. Be sure to select 1040ES:
https://www.irs.gov/Payments
