Making a good floor is one of, if not the most difficult thing you can do in modding. If you are going to attempt this you should understand it will be rather complex. Some things may also be very difficult, like making custom bosses, but good floors require these difficult things, like custom bosses and enemies. Otherwise, it's just like a vanilla floor.
Floors are split up into 6 basic parts:
Entrance, Tileset, Rooms, Enemies, Flows, And The Dungeon.
Entrance is what will allow the player to access your floor, it can be a pit, an interactable, anything like that. Without an entrance, your floor may as well not exist.
A Tileset is what sets apart your floor visually, you want people to be able to recognize your floor, not mistake it for some vanilla one. Rooms make up the basic composition of your floor, you should have quite a large number of rooms, specific rooms are not in predetermined positions, but rather random nodes at predetermined node positions.
Enemies help make your floor fun, new enemies with new attack patterns and such will make people actually want to play your floors.
Flows are what determine the layout of your floor, you have beginning nodes and make nodes branch off of each other to make the shape of your floor.
And The Dungeon is what contains a lot of info on the floor, like what its flows are, its health, multiplier, the tileset, etc.
A lot of different parts of the floor need to be built at the same time so read the pages in the order they are listed and follow their instructions if they say something like: "Do the entirety of X page, then return to here."
Fake prefabs are terrible when making floors (i dont actually know why, but i have been told so by people far smarter than me, so i will believe them), so it is a good idea to make an asset bundle by reading Making Asset bundles.