The way I plan out my stories is a bit of.. a mess.
I'd say that it all starts with a hook: an idea that looks so good in my head that my imagination starts swirling around it. Usually the hook is something visual: a character with a specific type of appearance, a scene that is a couple of seconds long, or sometimes it's just a name/a word that pops up in my mind and stays there because it wants to be turned into something bigger.
As I get my hook, and my thoughts start working on it, I decide to add details to the idea: say, I imagined a character with two robotic arms who is an underground fighter, now let's think how he lost his arms - he could lose them in a fight, or maybe he was an object of an experiment, and both of his arms got cut because something went wrong. If it was an experiment, what was the purpose of the experiment? It could be the making of a super strong or a cyber human. Why was this guy chosen for the role of the lab rat? Because he was an orphan, and there was no one to protect him. Who does experiments on little orphan kids, what kind of world is that? Let's build the world and give someone a reason to do experiments, then build up the main idea of the story.
From the main idea comes the purpose of a character's existence and their lore. How would they achieve the goal? What do they need to do it? How can I spice up their life path? Adding a nemesis? Creating a romance? Enemies to lovers arch? I can play however I want, building up a story that looks good in my head and is also logical, with a purpose to it.
If your hook is a scene, a name, or a phrase, the principle of this chain thinking is the same: what is happenning in the scene and what is the reason for it? who does this name belong to? who says this phrase and why? (you can actually have a lot of fun with names because they're literally white sheets, you can come up with anything to give it a cool name and then build castles around it)
Usually my mind creates a connection between the hook and the purpose. The purpose is the reason why I want to create this particular story and what I want to give my readers through it. It can be something philosophical or something as simple as just making up a cool adventure with, say, BL elements or an unexpected turn of events.
To plan your story means to explain everything to yourself in the first place. Starting off with a hook, give yourself a reason to make this hook into a story. Personally I often make stories to satisfy my needs, because no one can write stuff that I write (I don't think it's a toxic trait, it's rather something every author should have, aka 'confidence'). So if I want a story with this particular scene or this particular character, why not make it myself? Another reason can be sharing your thoughts with the audience or whatever comes to your mind - your reason is your motivation.
All in all, I prefer chain thinking to plan out the events: from the "middle" (which is, actually, the beginning of the story) move back (to the past) or forward (to the future) into the story to inspire yourself and explain what leads to what, and why your characters do what they do, then come back to the places that you didn't think through and finish building the skeleton. As of writing itself, you will have to decide which moment in the lore is the most suitable to start telling the story. Plan out the motives of your characters and remember the explanations you gave yourself. Start and move steadily to the end; adding stuff throughout the whole process is a normal thing, but you always have to remake the plan so that you don't confuse yourself and don't get lost in your own garden.