Congratulations! You have just finished your first draft. Now what? The internet is full of advice of what you should do, ranging from stepping away from it for some time to diving right back in to rewriting the whole thing. Whatever you do next is up to you, but one stage I would advice not skipping is the self-editing stage, even if you decide to use an editor at some point.
But if I'm going to make use of an editor's services, why should I self-edit?
You don't have to if you are willing to pay your editor more. Some charge you by the hour, others by wordcount, and all of that depends on what kind of editing services are required (I'll do another blogpost on those later). Self-editing can be beneficial in a number of ways:
You will catch plotholes, odd character development and those kind of things when you do (so your editor might not have to go through developmental edits).
Your editor might get through it faster and you could have your manuscript returned sooner (depends on when your editor of choice has a spot, of course).
You might save on money. Some editors charge differently for the different kind of editing stages (developmental might be more expensive than copy- or line editing). Some charge by the hour rather than the number of words.
You can do self-edits on any number of things in any number of ways. Keep in mind that nobody's process is the same, and that what I share here may work for me but perhaps not for you. At least, not in the same way. But if you have never tried it before, there's no harm in doing so if you wish to up your self-editing game.
When you self-edit, here is a list of things that may help you.
Print your manuscript in a different font than you wrote it. A different font will help just because your eyes and brain aren't used to it, so you'll catch mistakes faster.
Read your manuscript out loud (or have it read to you). This will help catch weird phrasing and dialogue that might not work.
Make a list of names, landmarks, cities, spelling, made-up words etc. (editors call it a stylesheet) so you can make sure everything is written in the same fashion (particularly helpful when you have massive worldbuilding going on and you're not a plotter).
Ask a critique partner to look it over for you. While they may not be editors, in their capacity as writers, they may spot mistakes you wouldn't. Critique partners may prove invaluable in more processes than the self-editing one.
If self-editing overwhelms you, do it in phases. Go for the worldbuilding, character development and plotholes first. Follow it by scenes that may not work, and only get to grammar and/or punctuation last (or leave that up to your editor completely if you feel too insecure about it).
Many authors choose to use programs such as Grammarly or Prowritingaid to help with grammar and punctuation. While they work well enough for programs, keep in mind they will never substitute an actual editor. On top of that, I have found you need to have a decent understanding of grammar if you don't want your manuscript returned with more mistakes than it initially had, and you don't want to accept everything blindly.
Don't be daunted by this process. You can keep going over your manuscript until you are satisfied. Hire an editor (highly recommended). Your book will get out there, and while that process is equally terrifying to the editing one, the feeling itself is amazing.