Your final design needs to be :-
Your 3D rendered final drawing could be drawn in ISOMETRIC or PERSPECTIVE ...
Your final 3D drawing needs to be rendered so that it looks like the material that you want your design to be made from.
You can use what ever medias you like to produce the rendered effect.
Cross sectional views of your design will show how the product is put together and where all the parts go and how it works.
Another way to explain your design is to show exploded views of your design or some parts of your design.