There's always something new to learn if you're a writer. You have to learn to write, to plot, to plan the book, to flesh out your characters, etc. You have to learn to keep a book bible. You have to learn how to write a query letter and how to pitch. You have to learn to work with an agent and how to read a statement. You have to learn how to market your book, how to ask for cover blurbs, how to conduct a virtual book tour, how to act at a book signing and work with booksellers. The list just goes on and on.
And here's one more that I'm going to add to that list, especially if you're not yet published.
You need to learn to edit. Now, this may sound strange coming from a freelance editor whose job it is to edit author's books. But you need to learn how to do at least the basics for yourself. If you don't, then you're doomed to pay editors to do it for you.
Learning the basics of editing doesn't mean you'll never need an outside editor. If you can't teach yourself, an editor can be a great teacher, in fact. But the more work you put in yourself, the less it will cost you to have a professional edit done. An editor often bases her fees on the time it takes to do the edits. If you do the small stuff, then she charges you less or she has more time to spend on the big stuff and on explaining why she's making changes in your manuscript. And that's important. If an editor just makes changes and doesn't tell you why, then how can you learn and begin to not make mistakes? So look for an editor who will work with you, not just for you.
It's easy if an editor says, "You're using too many -ly adverbs -- cut them and use stronger verbs." You can search your manuscript and change the problem. She might even change one or two to show you what she means, then leave the rest to you so you can learn and avoid making the same mistake in the future.
It's less easy if the editor says, "This passage bothers me. This is why. I think you need to rewrite this so that it does such and such." Trust me, it's better if she does this than if she rewrites it for you. You re-work it, send it back to her, and she says, "yes, that's much better." Then you've learned and you'll be a better writer for it.
6 months ago