Two ways to pay
The federal income tax is collected pay-as-you-go in one of two ways: withholding from wages, pensions, and certain other payments, or estimated tax payments made directly by the taxpayer four times per year. Self-employed taxpayers rely primarily on estimated payments, but a self-employed person whose spouse is a W-2 employee can use the spouse's withholding as a substitute.
Withholding allowance
Pub 505 walks through the IRS Tax Withholding Estimator and the new Form W-4 worksheets. The 2020 redesign of W-4 eliminated the old "allowances" concept in favor of dollar-amount adjustments, which makes the math more transparent but requires re-thinking long-standing withholding habits.
Estimated tax safe harbors
No underpayment penalty applies if total payments equal or exceed (a) 90% of current-year tax, (b) 100% of prior-year tax (110% if prior-year AGI exceeded $150,000), or (c) the year's tax minus $1,000. Most self-employed taxpayers use the prior-year safe harbor because it is a known number.
Annualized income method
When income is uneven across the year, the annualized income installment method on Form 2210 Schedule AI can dramatically reduce the underpayment penalty. Particularly valuable for seasonal businesses, year-end Roth conversions, and large capital gains taken late in the year.
Refundable credits
Refundable credits (earned income credit, additional child tax credit, premium tax credit) reduce the safe-harbor "tax" for purposes of computing the underpayment threshold. Pub 505 provides worksheets that account for these credits.
Annual updates worth tracking
Each year's edition of the publication updates inflation-adjusted dollar limits, references to the current-year forms, and any legislative changes from the prior year's tax bills. The "What's New" page at the front of each publication is the single highest-value page for an experienced reader; it flags exactly which paragraphs in the prior edition no longer apply. If you maintain a personal tax-planning workbook, building a discipline of re-reading just the "What's New" page each January catches almost every legislative change that affects a small-business return.
How to find what you need quickly
Reading an IRS publication straight through is rarely the right move; using it as a reference is. Open the PDF, jump to the table of contents, identify the chapter that matches your facts, and skim the worked examples at the end of that chapter first — they usually answer 80% of practical questions. The detailed rules in the body of the chapter then make sense in the context of an example. The publication's index, while less polished than a commercial tax-research database, is searchable in any PDF reader with Ctrl+F and surfaces the exact paragraph you need in seconds.