Our engine was designed from the start to use standard model formats X3D and VRML. This means that in principle you can use any 3D modeler to make models for your games, as almost everything can export to X3D or VRML (and when that's not enough, we also support many Collada, 3DS, Wavefront OBJ and other 3D formats). We consciously do not try to implement any 3D editor for our engine — it's a wasted effort, just look how magnificent e.g. Blender is, and how it's constantly improving. We want to use these tools, not reinvent them.
So, the basic guide to creating 3D data for our engine is actually trivial: grab Blender (or any other 3D modeller of choice), and export to X3D. You can try opening your models in view3dscene to see which features get exported correctly. In case of Blender, you can use our custom Blender X3D exporter (although standard Blender X3D exporter is also fine since some time), and see notes how does it work.
You may encounter some features that are not exported from your 3D
modeller in a satisfactory way. Fortunately, that's when the strength
or VRML/X3D appears: you can use
Inline to include one 3D file
within another, and you can simply write some X3D content by
hand. That's good for adding scripts to 3D data, and generally adding
stuff that is uncomfortable/impossible to design in your 3D modeller.
examples/fps_game/ data for comments,
especially the level file
Orientation: see Which way is up? chapter of the tutorial for in-depth discussion how you should orient your model. In short, use standard orientation of your 3D modeller, and the developer can make the engine adjust to it. If you use Blender, just use standard Blender orientation (+Z is up, follow the names like "top", "front" etc. in Blender viewports), and then export to X3D with standard settings (this will transform models to have +Y up, which is standard convention in VRML/X3D), and our engine will automatically pick it up correctly.