You just need to break the game world into smaller managed pieces.
For any enemies or AI characters, have a character editor.
For any decorative objects, a decorative object editor.
For any level layout or terrain, have a level editor.
Then in an editor (usually a level editor) include the ability to select and place everything.
It's important to also think about a way to designate areas of the level as being separate from other areas. That way you can designate which areas can see others. That way you can hide everything in areas you can't see, to improve performance.
Smaller issues like techniques for making floors, terrain, walls, doors are easier to focus on separately. A lot of times it's these elements that are created in separate editors like the ones mentioned above.
Creating these editors in Dark Basic Pro is useful since you can tailor them specifically to what you are trying to do, and you have easy access tot he source code when changes need to be made. The down side is that creating these editors take more of your time, and some of the capabilities you might need can be difficult to create. That's why most people surrender to using separate editors for things like modelling characters.
If you are using an editor that you didn't make, you can't make changes to the editor that you might need and the format of the information created might have useless information or be difficult to work with. This is why a lot of people and companies create their own in-house editors for creating levels, scripts, characters (except their geometry).
The philosophy behind most level editors in Dark Basic Pro are simple. Be able to create a game world by placing and manipulating objects, and be able to save enough information to be able to recreate the scene. Saved information includes what objects and where, light levels, textures and objects used, file path names. Storing information about characters in their own files (name, texture, object geometry file path) makes this process useful. This is also good for things like scripts, and other complex placeable objects.
This usually leads to lists of objects or other items. Arrays containing information on enemies, actors, objects, textures, path names.
