By having a tight "pucker" like aperture and by the amount of air used. The more air, the easier it is to play a high and low note. An average amount of air (don't know how to describe average) it's...
In the form it is currently in, brass... if you want to go back to before the horn was a horn, then copper.. bone... same as any other brass instrument.
French horns are made of brass. Some horns like mine however, are covered in a thin layer of silver for a nice look, supposed improve in tone, and protection.