I wrote this in response to a question on LinkedIn and thought others might be interested in it. The woman asking the question thought she could better edit her own work and then start freelancing as an editor if she took a course that was offered. She stated that she could not afford an editor for her 60,000 word YA book.
A writing course at a university is an investment.
I would not personally invest in a course that is solely about proofreading/ copyediting. Why? Because a good editing course (such as the one offered by the University of California San Diego Extension office) will cost over $1600. I am assuming if you can't afford an editor (for this novel probably around $600), you wouldn't be able to afford that either. Courses charging less than that may teach you how to track changes, but the rest of what you learn would only give you "tips" that will not be much benefit for a professional editor. Professional editors (and authors) need grammar lessons.
In addition, when you edit for someone, you use the style guide they want you to use (or if they don't have a preference, you use your favorite style guide). As an editor, I specialize in APA, Harvard, and MLA styles of editing. I use Strunk and White's "Elements of Style" for fiction. I am also familiar with AP and Chicago style manuals. I frequently have to review the style manual before I begin a project simply because they all have their quirks (especially when it comes to citation). If you are going to edit for others (or write for specific publications/ freelance jobs), invest in the style manual of what you are going to be editing and read it cover to cover. Then learn about the "track changes" and "comments" features of MSWord (you can find information online). Finally, take a grammar course to help you understand those quirks of the English language such as squinting modifiers, subject/ verb agreement, and faulty parallelism.
Proofreading requires you to not only know about the style manuals and grammar but also to know about formatting and the "styles" feature in MSWord (a formatting feature not an editing feature). Again, a basic course is not going to cover what you need simply because the key is consistent formatting. One document may have size 24 font for headings, another size 12 - as a copy editor, you are going to keep headings within the same document the same size, but the size may vary from document to document.
Even though I am an editor, I prefer to use 3 editors to edit my work after I am done. Yes, it is expensive, but it makes the difference between a professional and amateur product (book). However, I have also run into the problem of paying for an editor (especially on longer work). After you have gone through your work, you should use beta readers. I also recommend subscribing to Grammarly to check your work. It will actually teach you grammar while using it. In the end, you will need to set the work aside for a few weeks and then do a final edit. The goal is to get as many errors out of it before you make it public.
Also, if you want feedback (it can take 6 - 9 weeks) I am a reviewer on Reader's Favorite. Their website is very, um, toned down, but they will give you a free review on any book. You do not have to have your book published, but it is set up for published authors and targets self-published authors. I have reviewed works-in-progress, so I know people can submit them.