As we've reviewed each step of the editing process, we've narrowed the focus. The broad edit
examined the story arc and made sure all the parts fit. The deep edit
looked at the parts and made sure that they contribute to the story, and that every step has a purpose. Now we are ready to get down to the detailed edit, where we examine punctuation, grammatical details, spelling, and all of that nitpicking stuff. My advice for this step is controversial, but I swear by it.
Before a bunch of editors throw their red pens at me, allow me to present my defense. For anyone who has written a piece with this whole process in mind, let's look at this experience. This means the author has written it, rewritten it, reviewed it, and probably reread it a time or two. Then they promptly did a broad edit on the entire work, and then a deep edit. This means going through the writing at least five times and likely more. By the time a writer has been through a work that many times, they are no longer reading the words and seeing the punctuation - they are skimming it as their mind recalls the story. It has become so familiar that the details fly by, the mistakes now unnoticed.
For the writing workshop I run, I am relentless with the editing pen. I will review everyone's copy and mark up all the commas, spelling points, etc., making sure no subject-verb disagreement goes unpunished. And I can guarantee that the writers could no longer see those little things because their focus had been on the written piece - and rightly so. When they are the writer, the best role I can take is as their teacher, editor, and adviser. Conversely, when I am the writer, that detailed edit is best offered by someone else, because I have looked at my work way too much to be an effective editor.
Let technology help whenever possible. Most every word processor has a spellchecker, so let it do the heavy lifting there. More importantly, they often have grammar checkers. This is not as simple as a spellchecker, and a grammar checker can often be wrong. However, they give you a chance to look at a sentence in isolation and examine whether it uses the passive voice, it contains the wrong usage of their/they're/there, or whatever. Easily half of the stupid mistakes can be tracked down through simple, mindless technology, so use it before you drag someone else into your process.
Finding an outside editor is not an easy task, and should be approached with an open mind. If it is someone close to you, establish ground rules about what you are looking for. More importantly, keep an open mind and try to not factor in the existing relationship. Tell them you want advice on grammar, punctuation, etc., and keep an open mind. None of their corrections are mandatory, but whatever they notice should be approached with an open mind. After all, they are a fresh-eyed reader of your work, so if they think a sentence is clumsy, that is what any reader might think. Don't try to explain it to them unless you plan on explaining the sentence to everyone who reads it. Rather, consider whether a little reshaping delivers the message better. This is what their role provides, so take advantage of it.
If you hire an editor, all the better. Lay out what you want from the editor, set a price, and go through the situation beforehand, then let them do their thing. However, be prepared for a brutal review. At this step, you will feel that your story will only need a few commas or maybe a semicolon. The truth will likely be very harsh, and that's just fine. If a manuscript comes back soaked in red ink, think of each correction as a chance to learn something. Critique is brutal, but often critical to growth.
When I first started writing professionally, I kept a stack of my edited copy next to my computer, with all the red ink facing up so I always saw it. The pile grew over the years, this huge stack of shame; a permanent record of every mistake I typed. But whenever I needed a little affirmation, I would look through the stack. The papers on the bottom were thick with corrections, but as I flipped through the pages, the corrections became fewer and further apart. The simple mistakes no longer appeared, the complex problems did not show up as much. This pile of errors was now evidence not of my problems, but of my education as an editor.
As you progress as a writer, your talents as an editor will evolve naturally. You will develop a habit regarding the Oxford comma, your use of the passive voice will fade, and your writing will become tighter and more efficient. But to reach that point, the most important part is not constantly editing. First and foremost, you need to be a writer. You need to write, review, and rewrite. Don't worry about where the comma goes, that will come in time. Be a writer, and let an editor help you with the rest.