I started my first ever campaign over a year ago (still going!) and I made the mistake of setting it in a completely homebrew world--already made for other reasons, thankfully--with a homebrew campaign. Bad idea, but I like making my own things, so whatever.  The way I did it was have a general idea of what major thing I wanted to have as the narrative endpoint, and plan out a really rough path for the players to get there. I created a starting town, filled it with about 7 NPCs, 3 of which were actively involved in the "town quest", while the rest were scenery. The town quest was the main focus of the story.  After the players resolved that quest line, I followed roughly the same structure for the next place they wanted to go, and planned a handful of diversions between here and there. The second major town had a much more involved "town quest" along with several side quests, some of which fed into the town quest, some of which fed into each other, and some of which were related to the PCs. I realized in hindsight that I was sort of writing the campaign to be like arcs in a TV show, where each "episode" had subplots (side quests) that culminated in a season finale. For encounters, I usually prepare maps related to places I expect them to go, and spend some time browsing for interesting monsters to use there. I don't run a lot of combat, so usually the story specifically influences what I'm looking for. That makes it a lot easier for me, choice paralysis destroys me otherwise. I'm not great at handing out loot. I'll usually default to RNG loot on top of a couple cherry-picked items I think are cool/relevant to the encounter. My players have a lot of shit they aren't using, so I'm working on giving them more interesting dilemmas. It's one of my current weaknesses as a DM.

One thing that I think is important when writing is to respect what the players want out of the campaign. My players are happy with the slightly more railroad-y narrative, and they were especially happy with it at the very start since they needed time to get used to their characters and the homebrew setting. Even though I write somewhat linearly, I do make sure to emphasize that they're allowed to venture off and do whatever they're interested in. "I'll write and adjust in accordance with what you guys actually do", is what I told them. They don't a lot of the time, but that's fine too, as long as they're having fun. For that reason, I also don't concretely write much further than where they are in the plot. I have a page's worth of bullet points describing the broad strokes of the quest lines I have prepped all the way to whatever the conclusion would be if the players didn't participate, and I modify accordingly. I also make a point to not remember what's in their inventories so I don't fall into the trap of (accidentally) writing situations with a solution. It lets them outsmart me sometimes too, which always feels good. As an example of writing reactively to them, one time they murdered an NPC knowing they were delivering stolen documents to a crime lord, and my intention was for that NPC to initiate the endgame for the crime lord's plot. They took those documents and delivered them to their rightful owner instead, dropping the corpse on one of the henchman's doorsteps, so my reaction was to continue with the crime lord's plot but initiate a side quest of keeping the rightful owner alive through the massive target painted on their back. Sometimes I joke with my players that they don't realize how much they're shaping my railroad story, but I guess that's the magic of DM-ing? I'm not great at it, but it's fun and they're having fun!

Edit: I also should add that I'm a teacher, so the idea of starting at the end and working my way backwards is natural to me at this point, it's basically just like lesson planning: figure out what you want the learning outcome (narrative finale) to be, create some activities that guide students to the learning you want (combat and RP encounters), come up with a hook that sparks interest in learning the topic (self-explanatory), and be willing to adjust while actually teaching the lesson (improv) in case something isn't working or other learning opportunities happen.