The philosophy of Deliberate Practice applies very well to writing. There are some online writing courses you can take (e.g., David Perrell has one), but there's a lot you can do on your own (and with the help of some friends). Here are the two biggest things.
1. Start with an outline. This will help you cement the most important points of your writing/argument in your mind and enable you to start filling in the gaps. Often, I'll spend a lot of time getting the major points I'm making very crisp before I write a full draft. Doing this will also help you identify what's missing. The most important parts to focus on are (1) where a reader may disagree (i.e. where you'll need to make your argument stronger and back with evidence), and (2) where your reader will have questions that you'll need to answer.
2. Get feedback and revise 2-3 times. Getting feedback and acting on it is the single most valuable thing you can do; however, most people don't know how to provide writing feedback. They'll focus on grammar unless you specify what you want the reader to think about. There are two strategies here – (1) Provide your reader with a list of questions you want them to answer. I always use the following four: What did you learn? Is what you learned compelling (if not, why wasn't it compelling)? What didn't you learn that you wanted to learn? Where was it clear/not clear? (2) add specific comments/questions within your writing in places you are less comfortable with and want feedback.
I'd test out some of your more thoughtful and detail-oriented friends. You may find one or two who you feel are adding value and pushing your work forward. If you're going to pay for someone, it's very difficult to find a good editor. Most "editors" are only used to providing sentence-level feedback and may not have enough context about the subject your writing about to be helpful with content feedback. At Prompt, we find only 2% of the thousands of Writing Coach applicants we receive are good enough to make it into our network – i.e., the writing feedback skill is really hard to find.
1. Start with an outline. This will help you cement the most important points of your writing/argument in your mind and enable you to start filling in the gaps. Often, I'll spend a lot of time getting the major points I'm making very crisp before I write a full draft. Doing this will also help you identify what's missing. The most important parts to focus on are (1) where a reader may disagree (i.e. where you'll need to make your argument stronger and back with evidence), and (2) where your reader will have questions that you'll need to answer.
2. Get feedback and revise 2-3 times. Getting feedback and acting on it is the single most valuable thing you can do; however, most people don't know how to provide writing feedback. They'll focus on grammar unless you specify what you want the reader to think about. There are two strategies here – (1) Provide your reader with a list of questions you want them to answer. I always use the following four: What did you learn? Is what you learned compelling (if not, why wasn't it compelling)? What didn't you learn that you wanted to learn? Where was it clear/not clear? (2) add specific comments/questions within your writing in places you are less comfortable with and want feedback.