It really is trail and error until you find out what works for you--I spent...so many years starting comics and never really ending comics. So something that helped me out in figuring how to do that process for myself was to do a couple of short comics that weren't...my own stories. So I did a comic that was just some stanzas lifted from the Odyssey. I did another that used the text from the game Star Control 2. I did a comic based on Captian Novolin for kicks--like sometimes we backtalk fanfic--but it's a really good way to sort of piggy back and learn from what does and doesn't work from things that you enjoy (or in the case of Captain Novolin, wanted to do a parody of) And your not going to monetize off of them, so it's fine.
As for how to write a story in general, I honestly just recommend reading lots of books, both classic and modern, and as you read them, really taking note of why you liked them. Everyone writes and reads for a slightly different reason. I tend to write extensive lore, but my stories are really character driven. Other people are more driven by plot structure, building to big Christopher Nolan twists and world-building. Other people are writing satirically, other people are writing with intense meaning, other's are writing people making out. Everyone has different priorities.
For me it's like...it's kind of weird, but I have to draw my plot. Like with a pencil. I'm an artist, so I have to draw it. So I start by drawing my characters--which again are my favorite parts--and then I can visualize how they would speak and interact with eachother. I can draw where they live, I can fit world building into their settings, and their clothes. So my brainstorming looks completely different than someone else filling up tomes of digital libraries on googledocs, and it's probably because I'm a kinetic learner and a very visual person. So I'll kind of brainstorm like this, daydreaming different unwritten possibilities and conversations, until I have something that feels like a plot. Then, once I have a plot in my head, that's the point I write anything down.
So it's like...my process is kind of like pantsing, but I do have a pretty good idea of what I want before I sit down and write. But when I do an outline of what I think the story will be, it's usually wrong. I usually completely ignore my outline (and I also change my endings a lot.) My research is extensive, but I only do it for things I know will be in the draft, and I can only research after I do...at least some writing first. So I go back and forth between daydreaming (which is so important for me), drawing, writing, and researching until it's done.
But like the best advice I ever got for finishing a story, was to sit down and write a wordcount goal. So sit down and write 1200 words every day you can. That's not too difficult (and you can make it less or more). Never go back and rewrite things you've already written until your first draft is done. And just keep writing until you have some sort of ending. Then you can go back and rewrite what you need to tie the beginning with the ending. Again, everyone's super different, but that was advice I really needed, because I wasn't professionally trained to write, and so I was held back thinking I didn't know enough. What I needed was permission to make mistakes and just write till it's done.
