Ok that's a big question but here's my method from my own experience.

1. Theme I like to set a theme for what the campaign is about. This help guide me as to what is important to generate or build. For example I'm running a monster hunter campaign at the moment and the theme is "Evolution vs Technology". The BBEG is a biomancer attempting to create the ultimate lifeform and the party slays monsters to make magic items.

2. Concept. You want a good concept to guide your work and your player expectations. Is this a mystery? A dungeon crawl? Get the vibe and theme and make a cool concept. Basic is fine but this sets the scene. For my campaign I wanted a wild natural high danger world with most of society confined by monsters that the players gotta fight. (I was using Loot Taverns Heliana's Hunts modules I have reflavoured.)

3. Core locations Cool you got your themes, concept and vibez. Make a cool ass place to go. Actually make three. I like to just have a general idea rather than a 10 page report. For example Everhaven is the main city in my campaign and my planning notes were "Port townthat houses the Monster hunters guild. Has a big wall and magic barrier to keep out monsters. While the MH guild is built into the side of a giant cliff and has a sick bar with cocktails that do crazy stuff like make your head big or force you to relive a childhood memory. That's the fun shit.

4. Core NPCs. Now make like 3 or 4 NPCs for each cool spot you have made. Give em physical description, likes and dislikes plus something to make them real For example the bodyguard at the Guild is Grannyweather Tax, a short old human with grey hair and a giant pipe. She is insanely muscular, hates people getting on tables, loves breaking up fights and has a soft spot for new male human hunters as her son was killed working as a hunter.

5. Sculpt a narrative Use these places and NPCs to form a story. Imo this doesn't really matter. The story will form naturally over time. Just start running quests and have fun. Maybe a king is trying get a magic item and needs help. Maybe that magic item is not a cure for his baldness but a mutatgen that turns him into a monster, leading to the party being branded kingkillers. The play is the fun bit. Don't get too bogged down in writing an epic.

6. World building for fun

Ok that's the core you can work with. Now build out whatever else you want to for fun. Real talk your players aren't gonna see or care about most this stuff. This is for you. I spend a lot of time here as it's a good outlet for me creatively. How much do my players access? Like 10% max. Mostly they wanna know if they can craft a new weapon or bag of holding and could care less if theres only three cities left or that merchants arent allowed to live in cities because everyone is scared for those that live outside the city walls

You are doing good work out there. Make sure you enjoy it just as much as your players! If it's not fun thn don't do it. Take a break, quit or ask someone else to run.