It's simple really. I've been self-employed as a computer consultant for about 15 years or so now. What I do each quarter is send the IRS "at least" 20% of my gross income each quarter. Now the IRS has these worksheets you can use to figure your quarterly taxes each quarter. But in my opinion (and we know what opinions are like) it's a waste of my time. I just make my quarterly tax payments online each quarter at http://www.irs.gov/payments, print the receipt, file it for tax time and that's it.
Now if your gross income from all sources (not just your business income) is expected to exceed $160K for the year, then you'll want to send the IRS at least 25% of your gross business earnings each quarter.
Keep in mind that I'm talking about sending in a percentage of your *gross* business income, before you consider "any" business deductions at all.
With this method I find that at tax filing time I'm easily within $1000 either side of my total tax liability. An underpayment penalty is assessed if at tax filing time you owe the IRS more than 10% of your total tax liability, or more than $1000 - whichever is HIGHER.
For me, in the few years I've actually owed the IRS more, the most I ever owed in one of those years was around $475. In the years I get a refund, that refund is always less than $1000. So this works just fine for me.
However, if you want to do it the long way, the worksheets and instructions are at https://www.irs.gov/pub/irs-pdf/f1040es.pdf
Finally just so you know, I'm using the CD/Desktop version of the program. When I print my tax return after e-filing it, pages 3 thru 6 of the printout are the 1040-ES forms for the next tax year's (2019) quarterly payments. I don't have a choice on that. If (and as far as I can tell "only" "If") I owed the IRS taxes in the currently filed tax year (which now is 2018) then the program automatically generates the 1040-ES forms for each quarter.
Also, be aware of the quarterly tax payment deadlines too. For 2019 they are:
1st payment . . . . . . . . . . . . . . . . . April 15, 2019
2nd payment . . . . . . . . . . . . . . . . June 17, 2019
3rd payment . . . . . . . . . . . . . . . . .Sept. 16, 2019
4th payment . . . . . . . . . . . . . . . . .Jan. 15, 2020
For the "final" finally, if your state taxes personal income, then you will also need to send your state quarterly tax payments. Since states tax at a different rate than the IRS does, I suggest you send your state each quarter a percentage of your business income that matches the state's rate of taxation, and you'll be fine with that.
