Start with the form instructions
Every IRS form has detailed instructions explaining what each line means and how to complete it. The Schedule C, Schedule SE, Form 8829, and Form 4562 instructions cover most small-business questions. Available as PDF on IRS.gov.
Check the relevant publication
For deeper guidance, the IRS publications (Pub 334, Pub 463, Pub 535, Pub 587, Pub 946, etc.) explain rules with examples and worksheets. Read the table of contents to find the section addressing your question.
Read the Internal Revenue Code
For controlling authority, look up the relevant Code section. For example, Section 162 governs business expenses, Section 179 governs immediate expensing, Section 274 governs travel/meals/gifts. Code text is available free at LII (Cornell), USC.gov, and IRS.gov.
Treasury regulations
Regulations interpret the Code with much more detail. They often resolve gray areas. For example, Reg. 1.262-1(b)(7) discusses the personal vs business use of clothing; Reg. 1.183-2 lists the nine factors for hobby vs business. Available free at the same sources.
Tax court cases and IRS guidance
When the Code and regulations don't resolve the question, look at IRS published guidance (revenue rulings, revenue procedures, notices) and tax court cases. Free sources include CourtListener and Google Scholar. Paid services (Checkpoint, CCH, Bloomberg Tax) make the research much faster.
Common mistakes worth avoiding
The recurring mistakes filers make on this topic cluster in three patterns: (1) optimizing for current-year tax at the expense of multi-year tax, (2) treating the choice as binary when the IRS framework actually allows nuance (partial elections, hybrid methods, year-by-year reassessments), and (3) deferring the analysis until the return is due rather than running it during the year when the result can still influence behavior. A short annual review — even thirty minutes — catches all three failure modes and replaces vague intuition with documented reasoning.
When to bring in a professional
DIY tax software handles most small-business returns competently, but a handful of situations reliably justify a CPA or enrolled agent: an entity formation or election, a multi-state filing situation, a significant fixed-asset purchase that triggers Section 179 or bonus depreciation modeling, a retirement-plan setup, an IRS notice or examination, and the year of an entity sale. Outside those situations, software plus an annual half-day of personal review produces a defensible return. The cheapest professional engagement is a one-hour consultation rather than a full-service tax-prep relationship.