There are a few things that are consistent across all kinds of copyediting. No matter if you are editing books about tatting lace with dental floss or curing cancer, no matter if you are losing your mind because of nonediting drama, you must follow these rules.
1. Be consistent.
Copyeditor, thy name is consistency. Very few people will notice that on page 41, “yellow-throated warbler” isn’t hyphenated. [And, no, I’m not sure there is such a thing as a yellow-throated warbler. I don’t care. If there is, it should be hyphenated.] But one person might notice, and it probably will be your managing editor, and that would suck. This is what find and replace is for, people! Also, if your managing editor does not approve of your editorial choices, at least you can claim consistency.
2. Would you bet [your favorite person in the world’s] life on it?
If not, look it up or double check. I mean, I’m pretty sure that the author’s name was spelled “Machacek” on the previous page, but then I think about whether I would swear on my grandmother’s grave that Machacek doesn’t have an extra letter I overlooked, I go back and check. Grandma was interested in journalism. She’d understand.
Are you sure there should be a semi colon before “whereas”? Pretty sure? Kinda sure? Not good enough. Go to the Chicago Manual of Style and look it up.
3. It will always take longer than you think.
What are your immutables?