Editing an article has four parts. I have described them in the order followed.
Structure Edit, Find the article’s angle. What is the central theme the author wants to communicate. For instance, in an analysis article of the story Yellow Wallpaper, the writer chose to illustrate how the story was not about mental illness as is popularly assumed, but gender inequality and the tendency to see women as inferior irrational beings.
Once you discover this author angle, things become easier. List out various points made by a writer, and group them into categories. That can then be ordered into a nice flow. For instance, in an abstract written for a conference topic on technical writers making videos, the abstract began with why video making is important to writers these days, then the various skills that the conference session would try to cover, and what exercises would be conducted.
Content editing requires the editor to look at each point with a keen eye. Facts that look dubious are worth a quick check. Of course, you cannot and should not fact-check as much as the writer, and the idea is to only look for red flags, or something suspect. Repetition of points is another thing to look out for. Although this should not include repetition for the sake of poetry or emphasis. You can also suggest points that may go with the article’s theme.
Style Edit, looks at the style of the overall article. Could you have started better, perhaps with an anecdote (that you can make up) or a quote. If using an anecdote to start, reference it at the end as well to neatly tie up the article. Try to add narrative turns that lead from one paragraph to the other, a sort of hook to the next paragraph. And ensure that persons and entities are referenced in a tidy manner. Avoid language that sounds pompous, and sentences that sound wordy: the idea is to communicate things in the shortest and easiest sentences possible. Try to use evocative words, or words that trigger a nice image or visual, like guzzle, heap, scoffed, although do not go overboard.
Presentation Edit This is all about nitty-gritties like spelling, grammar, and headlines. Check if the headline is strong, and gives insight into the article and perhaps even dramatises it (Who actually won Independence for India?).