Editing is a professional activity and learned through experience. The things that editors check can be lengthy checklist. We can however summarize four major areas to keep in mind while dealing with any piece of content, whether a blog post or technical report.
The editing process can span across minor punctuation corrections to large structural improvements.
4 major areas of content editing
1. Spelling, punctuation, grammar
The first thing you must look out for are the basics of formal language. Cleaning this up should make the content clean and trustworthy.
2. Style: consistency
This does not refer to the style of writing, but consistency in the way we use capitalization, money and units of measure (USD or $, per cent or %), abbreviations, designations, dates, etc. Every publication may have its style guide set. It is a good practice to adhere to one fixed set of rules. Professional editors are also well acquainted with popular style guides such as the Chicago Manual of Style, APA or that of the Economist.
Structure and outlines are usually an aid to help readers understand. If you notice disconnections, jumps and inconsistent flow of paragraphs or sentences, feel free to put it in the right order or flag it with the author.
4. Substance: economy of expression
Brevity is the essence of good writing, whether prose or verse. Repetition and redundancy are like weed in a garden. Good editors always keep an eye out for economizing on words and expressions to convey just the right meaning. Choice of words often goes wrong in first drafts, and the editing stage is where these are corrected.
Finally, the content (article, blog, thesis or dissertation) should sound like a rounded whole, with a proper beginning, body, and conclusion, all interconnected. It should be easy to browse and follow for a layman, and interesting to those with more knowledge of the subject (by giving specifics and details at appropriate places).
To know more about the specific issues and problem areas that professional editors attend to, refer to this excellent guide: