Even if you’re submitting your novel to a traditional publisher, hire an editor first. It’s the difference between putting your work out there as a DIY project or as a product created by a professional. Think about it: You worked hard on this, and invested maybe years at it. I guarantee you, no matter how good a writer you are, you missed something. It might be something important (what happened to character Y? why is it night now? I thought the border with country X was over there? how can Z know this when only W saw it happen?) or it might be a lot of little things (you used “bleak” three times in that paragraph; these three characters have names so similar I can’t keep them straight). No matter how sure you are that you’ve made your Deathless Prose perfect, hire an editor to look at it with professional, unbiased eyes.
Even if you believe your Deathless Prose is so, so good that you don’t need no stinkin’ editor, at least hire a copyeditor to make sure you used all of the words correctly. You don’t want to be the author who kept using “effluent” when you meant “affluent.” Trust me: Readers will notice, and we will laugh.
Don’t skip the last step.
A disclaimer: I am not a fiction editor. I’m a copyeditor and a managing editor, which means I can make several hundred pages of raw Word document into something that reads well and fits into the space and formatting restrictions allotted, but I’m not an expert on why a particular chapter doesn’t work. I can, however, tell that is doesn’t work for me, as a reader and as someone who cares deeply about the quality of self-published fiction.