Here is a typical process that good editors use:

1. Scope the job—and be specific. This means essentially determining the size of the job. Personally, I determine the scope using word count. But however it is done, a good editor will have a specific procedure, so as to be able to give an accurate estimate of cost and schedule up front.
2. Perform a “first reading” of the text. During the first reading, a good editor will be looking for many things, from sentence structure and punctuation to appropriate word choice to confusing content. Unfortunately, this complexity means that he or she can’t help but miss a few things. That’s why the next stage in the process must be…
3. Perform a second reading of the text. A good editor would never rely on just one reading. If you don’t believe in the value of a second reading, try it sometime with your own material. The second time through, I guarantee you’ll catch lots of stuff you missed on the first reading.
4. Check the in-text citations against the reference list. This is not an insubstantial task, especially in scholarly work. The reference list may contain 100 references or more, and every one of them must be formally cited in the text. Similarly, every citation must have a corresponding reference in the reference list. A good editor can make sure that everything is consistent, or let you know what is missing.
5. Format the in-text citations and the reference list. A good editor will query the author about the style guidelines to use or will download the guidelines from the Web site of the journal or publisher to which the author is submitting the document. This stage can sometimes be quite time consuming. However, a good editor will do it gladly because it means that the piece will be in the preferred format for the client’s journal or publisher.

So, you can see that editing is both a science and an art and that there are numerous steps in the process that must be worked through if a high-quality outcome is to be obtained.