Writing fiction is a lot of fun, although sometimes it’s hard to keep it that way. I hesitate to give specific advice since different techniques work for different people, and you won’t know what will work for you until you’ve started writing. But I can say what worked for me:
(1) making a routine (x time spent trying to write every y)
(2) keeping a notebook with ideas (or a phone app or whatever)
(3) making friends who are also writers
(4) cultivating a group of people to beta-read and give you feedback
(5) researching writing craft so you know what techniques are out there
Personally, I like to write outlines detailed enough that I won’t paint myself into a corner, but vague enough that writing feels like a process of discovery. Other people prefer to just start writing and then do revision after revision to iron out the problems later — still others make their outlines more and more detailed until the outline becomes a novel. You can produce a good book from anywhere on the outline/no-outline spectrum, but the question is how to have the fun possible during the process of writing.
I start by collecting all my notes about a story and finding connections between them. In parallel, I have a basic story structure (e.g. The Hollywood Three Act Structure, but there are many, and I try to use different ones for each story) and I plug in my ideas where they fit into the structure. Then I look at the beginning of the story and the denouement and try to figure out what needs to happen between them.
I ask myself: what do I promise at the beginning of the story? What’s the most interesting way to deliver it? What’s my central message or theme? How does the theme get developed? What do I have to teach the reader in order for the denouement to make sense to them? Most importantly: what does the protagonist want? What mistake is the protagonist making that stops them from getting what they want? How does the protagonist change in order to get what they want?
I need to make sure that each thing that happens to the protagonist (the plot of the book) makes that person change in some way.
I’m still working out the kinks in the process.