Proofreading someone else's writing puts you in an unusual position. You're responsible for catching errors, but you're not the author. That distinction matters more than most people realise when they first pick up someone else's document. The temptation to rewrite, tighten, and restructure is real. Acting on it is usually a mistake.
The job has two parts: find what's wrong, and leave what's right alone. Getting both right at the same time is a skill, and it takes a different kind of discipline than proofreading a document you wrote yourself.
Understand what you've been asked to do
Before you read a single line, clarify the brief. "Give this a once-over" and "proofread this before it goes out" can mean completely different things to different people. Ask whether they want errors corrected, or whether they also want feedback on clarity and structure. Those are two separate tasks. Conflating them is the most common way a proofreader damages a working relationship.
If they want errors only, your job is grammar, spelling, punctuation, and consistency. Full stop. If they want a broader review, agree on that scope explicitly before you start marking up the document.
Separate their voice from their mistakes
This is the hardest part. Every writer has habits: preferred sentence rhythms, pet phrases, a tendency toward short sentences or long ones. None of that is your business unless it produces an actual error. A short, punchy sentence isn't wrong because you'd write it differently. A comma splice is wrong because it's a comma splice.
Train yourself to ask, before flagging anything: is this an error, or is this a choice? Repeated short sentences can be a stylistic device. An unconventional word order might be deliberate. If you're unsure, query it rather than change it. A question mark in the margin does far less damage than an unexplained substitution.
Work through the document systematically
Don't try to catch every type of error in a single pass. You'll miss things. Instead, read through the document at least twice, each time with a specific focus. A first pass for meaning and flow tells you whether sentences are doing what they're supposed to do. A second pass for surface errors catches spelling, punctuation, and grammar. A third pass, if the document is important, checks consistency: headings, capitalisation, number formats, and terminology.
That last category is often where inconsistent capitalisation in business documents hides. It's easy to miss because readers naturally smooth over inconsistency when reading for meaning. Slow down and read for pattern.
Mark changes clearly, and explain them when they're not obvious
Track changes exists for a reason. Use it. Never silently overwrite someone else's text without a record of what you changed. The author needs to understand what you altered and why, both so they can accept or reject the change and so they learn to catch the same error themselves next time.
For changes that aren't self-explanatory, add a comment. "Changed to active voice" or "comma needed before dependent clause" takes five seconds to write and saves a frustrating back-and-forth later. For anything you're genuinely uncertain about, use a comment rather than a correction. "Is this the right figure? Check against section 3" keeps the decision with the person who owns the document.
Know where errors tend to cluster
In any document, certain sections carry more errors than others. Transitions between sections are one. The final paragraph is another: writers often rush the ending. Headers and captions are a third, because they're formatted differently and proofreaders' eyes tend to skip over them during a general read.
Numbers deserve a dedicated pass. A wrong figure in a proposal or contract can cost more than any grammatical error. When you're proofreading someone else's document with data in it, check the numbers against the source if you have access to it. If you don't, flag every numerical claim for the author to verify. The approach used in careful work on proofreading numbers and data in business documents applies just as much when you're reviewing someone else's work as when you're checking your own.
Resist the urge to rewrite
You'll find sentences that are technically correct but clunky. Resist. Your job isn't to make the document sound like you wrote it. If a sentence is clear, grammatically sound, and free of errors, leave it. If it's genuinely ambiguous in a way that could mislead the reader, raise it as a query, not a correction.
The same discipline applies to word choice. If the author has written "utilise" and you'd write "use," that's a preference, not an error. Flag repeated jargon only if it's likely to confuse the intended reader. Otherwise, let it stand.
Return the document with a brief summary
When you return the proofread document, include a short note explaining what you found and how you handled it. Three or four sentences is enough. Note any patterns you spotted ("there were several places where commas were missing before dependent clauses"), any sections you flagged for the author's attention, and anything you weren't sure about.
This note does two things. It gives the author confidence that the document was reviewed systematically rather than skimmed. It also gives them a chance to spot if you've misunderstood something about their intention before they accept your changes wholesale.
Proofreading someone else's work well earns trust quickly. It's also a rare skill. Most people who offer to proofread a colleague's document end up editing it instead, which is a different job that nobody agreed to.