It is my basic assumption that you will be teaching this concept to several elementary (or at least beginning) students at one time. I also assume that right now you have a flute (in its case) in front of you and that you will follow along step by step to carefully learn this concept.
THE HEAD JOINT
Do NOT put the flute together yet! Have each student pick up only the head joint, and holding it with both hands, place the embouchure plate securely in the natural indentation of the jaw. Ask the student to be sure he feels the lower lip covering 1/4 to 1/3 of the embouchure hole. If not, move up and down as necessary to achieve this feeling.
FORMING THE EMBOUCHURE
Now, demonstrate, using your own facial expression, by making a silent "WHEE". Ask each student to do so. This should pull the corners of the lips outward and flatten the lower lip which is resting on/over the embouchure hole. Immediately ask the student to add a "too" following the "Whee". WHEE-TOO should (1) set the embouchure and (2) give the feeling of properly starting the first tone.
THE FIRST TONE
Discuss the theory that air will be directed INTO the hole, NOT ACROSS it. The more directly the air goes into the hole, the more likely a clear and pleasing sound will result. Use the airstream as a pointer towards a spot on the backwall inside the the embouchure hole. (You can point to this area from the outside.) I call this FOCUS and use that term from beginners on. Only with FOCUS can a beautiful flute tone result.
Take a deep breath. Re-form the embouchure (Whee) and blow (too). Demonstrate and ask each student to produce a tone. It should be A2 (second space A).
Testing each student you will encounter one of five predictable results:
#1--CORRECT RESULT More often than not, a reasonable tone results. Imitation of your tone and pitch will be automatically attempted. When it happens, it produces A2 and should sound like this:
#2--NO TONE, RUSHING AIR Air is going across, not INTO the hole.
#3--SOME SOUND-MUCH AIR Air is rushing over as well as into hole (splitting the tone) and out the sides of the lips. "Whee" embouchure is not being maintained.