Through my own writing, especially my novels, I have discovered the importance of thorough proof-reading. No-one's work is foolproof. Reading and re-reading your own work, over and over, is mind-numbing and can induce a kind of 'word-blindness'. Having someone else, me, to check it, is the answer.
No matter what the source material, I begin proof-reading the same way, covering the basics such as spelling and grammar, and then move on to more in-depth checking.
General checks
Spelling * Grammar * Punctuation * Repeated words * Copy and paste accidents * Missing / Mixed-up sections * Formatting
Advanced checks
Language * Regionalisation / Localisation - Or should it be regionalization? * House style * Slang * Jargon * Table of Contents / Indexes / Page-numbering * Brand-names * References * Abbreviations and Acronyms * Emojis / Web syntax * Readability * Flow
Tools
If time is extremely tight, I will use various tools to make a quicker but perhaps less in-depth analysis, such as Hemingway or Ginger.
Proofreading Marks and Symbols
If I am proof-reading a hard-copy of a document, I can - if you wish - use standard proof-reader's marks to indicate issues. For online or electronic copy, which is the vast majority of material these days, I use simpler highlighting tools.
Additional on-request checks
Re-writing - If there is a need for significant re-writing, this is a separate service that I can provide on request.
Fact checking - I will also undertake Fact Checking - which can be a time-consuming task - on request.
Layout / Device Checking - On request, I can check that text and formatting flows correctly on various devices and at different screen widths. For example, does simply orienting a screen to landscape mode break the text flow? Is hyphenating working?
Screen-readers - Sometimes text is semantically incorrect for screen-readers, and text can often be 'hidden' in unreadable graphics, rendering the site unusable for some people.