I'm in the midst of writing a one shot that is linked to a homebrew module that my friend is running at the moment. I'm in two minds of whether to have it set in the same reality, or have the players go into a different reality to give myself a bit more creative freedom.

When you are writing a campaign, how much content is your own vs how much do you borrow from D&D content?

Do you have any tips for combining the two, alterations or just general tips for a newbie world builder?

I am in the early stages of doing this and haven't fully decided what I want to do. For me I am a huge dragonlance fan and want to put a little of that into the world I create without having it be too much. Interested in seeing what everyone else is doing too. 

Usually my mix is about 80% official and 20% homebrew depending on if there is a reason for homebrew things like items or monsters.

I have a pretty varied blend - I usually have my ideas/concepts/themes in place of what I want my world/campaign to look like and do a bit of research into them. If there is official content that aligns with my vision for the world building I use it. If it doesn't fit with the world, I take inspiration from it, cut and paste what I like, ditch what I don't and then fill in the blanks with my own creations.

The official content is there to be used but you don't let yourself get bogged down in it all; you don't need to make every piece of official lore work in your world. At the same time, you don't need to reinvent the wheel, so where-ever you can make your job as DM easier, take it. 

I've also 'borrowed' npcs from Dimension 20, and concepts/themes/even scenes from horror movies and just twisted it slightly to fit into world. Don't feel pressured to make everything in your world brand new and unique. What makes it new and special is the way you tell the story, and the way your players walk in the world. 

The only time I had a problem with this patchwork approach was when a player was super into DND official lore and said something to the effect of 'that's not what the source book says', which is pretty easy to counter with 'yep, as you know, we're playing in my homebrewed world.'

For world building in general - my biggest tip is have a clear plan for the basic structure of your plot line before you go too in-depth about the world. From your basic plot line and all the major story beats you want your players to hit, start to build the world out around the plot line - don't waste your effort writing the history of a town that your players will breeze by. If you focus on the micro before the macro, you can start to feel burnt out and also frustrated with your players who may not necessarily engage on that level. 

Over time your plot line will shift and change to match the world that grows organically, as well as to your players interactions with the world. Be flexible with the parts of the world you can be flexible with, but make sure to stay true to the very core concept/theme/plot point that started the world building. 

When you create towns or cities - ask yourself what makes that place unique or special. Is it a port town, is it a farmer's hamlet, is this where the city guard do a years rotation prior to becoming knights? How large is the city and what kind of districts (if any) does it have? From there, ask who are the likely people that would live in this city eg officials, shop keepers, town gossips, travelling merchants. This is a pretty quick way to build out a city and quickly give it a unique feel. 

No matter what you do, your players should appreciate the time and effort you take to create a fun, enjoyable and unique experience for them! Best of luck with it all!

For my stuff, it depends on what my players want.

The vast majority of the time, they want what comes out of my head, lol, because I tend to make stuff they enjoy. As a result, my mix is usually 90 to 98% "homebrew" and very, very little d&d stuff.

One of my biggest complaints is that they put too much lore into the mechanics, lol. (note that most people want the opposite of me).

That said, I will also note something important -- when you are deciding what you want to use or drop, do it for the sake of the kind of game you are playing or plan to play, not because of where it comes from. My lack of use of official stuff isn't because I have any distaste for it -- it is because it doesn't fit or requires me to change what I want my world to be like.

One of my core worldbuilding principles is that the world does not bend to fit the game, the game bends to fit the world.

i like to start out with an official campaign and assume it's headed to the second chapter. but there's always this point (especially if they have to travel to another location) when the players seem to question whether this is a fun direction or not. and they'll stick with it if you will, but i strongly recommend testing their faith on that. later they'll be fast traveling but right now they're walking and so much can happen when you're walking. so i toss in some distractions on the road and if they have so much fun they forget the not-yet-epic quest they had promised to maybe perhaps look into if they had time, then that's plenty fine. it certainly helps to have read up on some official one-shots from the anthology books in case they seem interested in a Keys From the Golden Vault heist or a Tales from the Yawning Portal dungeon. its just so much more worth my time to edit existing stuff and fill in the gaps than it is to create a whole new Exandria. your mileage may vary.

I use an older, unsupported setting (Mystara), the world is pretty much as it appears in the books, and I allow most of the official subclasses and many of the official races/species, but I have augmented it quite a bit too. Most of the “typical” species (elves, humans, goblins, etc.) have their own custom shared background, and I have added a shedload of homebrewed subclasses & species to it too, and a lot of homebrewed spells, magic items, and NPCs & monsters too.

My suggestion regarding altering official content and/or combining it with homebrew, use whatever official content fits for you and your ideas, and scrap the rest. It’s really just that simple for the most part. Just be sure to be able to explain your changes to your players so they don’t end up confused.