The quarterly estimates are optional to pay but if you owe too much on your tax return then there is a penalty.
The first year you don't need to pay estimates as long as you pay in (by withholding) as much as your tax was last year. But if you will have a big income you should send in estimates so you don't owe too much next April on your tax return.
For SE self employment tax - if you have a net profit (after expenses) of $400 or more you will pay 15.3% SE Tax on 92.35% of your net profit in addition to your regular income tax on it. So if you have other income like W2 income your extra business income might put you into a higher tax bracket.
You must make quarterly estimated tax payments for the current tax year if both of the following apply:
- 1. You expect to owe at least $1,000 in tax for the current tax year, after subtracting your withholding and credits.
- 2. You expect your withholding and credits to be less than the smaller of:
90% of the tax to be shown on your current year’s tax return, or
100% of the tax shown on your prior year’s tax return. (Your prior year tax return must cover all 12 months).
You can send in some for the 4th quarter right now before Jan 15 so you don't owe do much in April on your tax return. You can pay directly at the IRS,
https://www.irs.gov/payments
