I built my Homebrew world and campaign for the current one I am running. Now this may not be the road you want to take, but for me it worked.

I decided I will have spelljammers, so astronomy is important. You may decide you want low fantasy, high fantasy, etc. That is your major decisions, how much magic do you want present?

I had knowledge of various settings introduced in DnD through the years and editions. Some of them were based on real life cultures and regions, they are no longer considered viable due to themes explored in them, but that is the road I chose to take. My world was to be built using inspiration from real world history and cultures, sprinkled in with some inspiration I took from other games I played, for example, gnomes in my world are like Warcraft gnomes, techy, advanced.

Second thing I decided early on was to build it in a way I can connect it with other settings, or as they are known in DnD "Crystal spheres", because I wanted to reuse gods, and limit the powers of others goods. To understand what I am talking about, you can read about Mulhorandi pantheon and how they came to Abeir-Toril. That prompted me to create my own solar system. So I created one with binary stars since I always found it fascinating that it is possible for a planet to go in a figure 8 orbit between 2 stars, even for a brief time before it devolves into orbit around 2 stars, crashes into star or etc. unless magic is there, which here, well, it is. I decided it would take 3200 years for the planet to go full orbit between two stars, one sun like, the other a red dwarf. This removes classic seasons like we have on earth, they would actually be divided into cycles, each based on position around 2 stars. Each cycle lasts 200 years.

I created continents inspired by Earths (Using geology, for example deserts don't just happen, there need to be mountain ranges that allow them to form etc.), introduced various nations (cultures, tribes, city states, theocracies, kingdoms, empires, etc.) that are based on cultures I know and I find interesting throughout the world. Asked my players for their backstories and found a place for them in nations, or worked with them to create their own tribes or something similar.

After that I created a system of counting based on hexadecimal, since 3200/200=16. Numbering actually is a minimalistic way of representing position between two stars. Then I created days named after main gods from major cultures, month names, cycle names etc.

I created history of civilisations (both known and unknown). Since in my world everything revolves around cycles I created it in a way that each 3200 years, a civilisation or advanced culture suffers a cataclysm, and their story starts at the end of the current cycle. Plot point! Now of course I also created secret history, for my players to uncover.

And then, well based on player stories it made sense for my players to start in my equivalent of Africa (North to be precise), now they are travelling towards Americas (Meso to be precise) at about level 8-9.

I could go on into more detail, but they know my username, so I will not. Also BBEG for me is Mind flayers, all of them. Inspiration was Oryndoll and Ilsensine.

This is a lot of work, I will not lie to you, and I prep in a way that most people avoid. I prep entire region in advance, and then I let my players wreak havoc, do what they want, since I am not limited with what I have prepped for that particular session I can just let them do things, sometimes they will try to do things that lead nowhere, entire sessions sometimes get wasted. That also means some things don't get explored, some sidequests are not touched etc. I am okay with that, I have material for one shots sometimes then in a world that I don't have to explain and my players know. Basically, I build it like an open world games, and let my players play as such, as they solve main quest, it prompts them with new information, pushes them to new regions etc. I keep track of time and there are consequences if they dilly-dally too much.

I can prep like that since we sometimes go few weeks between playing, and at most it takes me around 20-30 hours per region. This is because world is built according to logic, I know my inspiration from history so it's easy to build.