When you create 3D objects, create them to be life size. Full-size objects are easier to drop straight into your experience.
All objects should face the same direction. Use a right-handed coordinate space, where +Y is up, +X is right, and -Z points forward from the origin.
When you model an object for your scene, make sure to place it on the ground plane at the geometric center of the object base.
Remember, 3D objects can be viewed from all sides. Use complete objects, and be sure to render all surfaces, even those that a user might not immediately see, like the back surface of a curtain, or the bottom of a couch.