Ideas come first. Well, really, the first thing is: what excites you?
It doesn’t matter if it’s a character, a twist, a concept, or just a single moment. The thing you’re looking at and cannot wait to drop on your audience. It’s the one thing that will drive you to commit to the project, and will carry you through the parts that tie it all together.
Once you’ve found that, you should have an idea of where to start. Everything else is negotiable.
A formal pattern would be: concept, lit review, preliminary plotting, research, plot refinement, write, rewrite, rewrite, rewrite, rewrite, rewrite… call it close enough to done, rewrite again.
Concept is easy: I want to write this. At this stage it’s fairly nebulous. You can probably scribble out a concept in a single paragraph. And almost any writing prompt you’ve seen falls into this general category. This is just having the idea.
Lit review is where the work starts. You need to go out, find other material in the genre, read it, and start building a comprehensive idea of what the genre you’ve just waded into really looks like. This isn’t always a necessary step. If you’re a hard core sci-fi fan, then a lit review of space operas probably isn’t going to be that useful, because you already know what you’re looking at. But, if you’re wandering into new territory, this is absolutely vital, because you will learn new things about the genre. You should keep an eye on what works and what doesn’t in the genre as you’re going. This will help you avoid easy mistakes, and will help a lot.
Preliminary plotting has probably already started. You might have actually started this when you were building the concept. So if this is already done, you can move on. This boils down to, I have characters that do these things. You might only have a vague idea of what happens along the way or where they’re going, but you should have an idea of where your story starts.
This is probably where you’ll start to get a handle on your characters. You might have had some concepts for them already, but after your lit review, you should have a vague idea of who you want in your story. You’ll probably keep refining them through the entire process, and research will tell you a lot about who they should, or would, be.
Research is a lot like the lit review, it’s work, and you probably started doing research during your lit review. This is going to be very dependent on your preliminary plotting and what you learned during your lit review.
One piece of advice about research: buy your books and keep them. You never know when a stray book on Arthurian lit or a history text on Mesoamarica will suddenly become relevant to what you’re doing right now. Also, if you’re in college, keep your textbooks. I know it’s basically free money, but good ones can be incredibly valuable resources later on. Being a writer requires being a book hoarder.
Also, I know I put this in a linear order, but, research never ends. You do the research you want before you start, but throughout the project you’ll keep hitting points where you need to go back in. Again, if you still have the books from earlier they’re still available as resources.
Plot refinement can be just nailing down the order of things in your head before you start, or it can be sketching out a formal outline. It depends on what works best for you.
Rewrite. Because, as they say, “writing is rewriting.” The hard part here isn’t actually finishing, it’s knowing when to stop. Once you get going, odds are you’ll never be completely satisfied with what makes it to the page. But, if you don’t check yourself on this, you can easily end up spending ten years revising a project to death. Don’t do that.
Obviously, you can rearrange these however works for you. Generally speaking your lit review and research will inform things about the story you’re trying to tell, closing off options and opening new ones, so they should come first. But, honestly, if you have something you need to get on the page, do it. You can always clean it up, fix it, or feed it to a grue later.
Two things to grab:
Steven King’s On Writing. I’ll admit, I’m not a huge fan of King, but his advice here is excellent.
King recommends of reading Strunk and White’s Elements of Style. Do that too.