Preparation and Buzzing
I'll make an argument here for the need to at least attempt a few minutes of buzzing the lips before using a mouthpiece or horn. It is important for students to understand that horn playing, like singing, depends on a vibration generated by the player and not by the instrument itself. Therefore, the quality and reliability of the tone is more dependent upon personal perception and reflexes than it is upon "equipment." Any effort directed toward this understanding, including- five or ten minutes of "buzzing" or attempts to do so, is a valuable- able starting procedure.
Buzzing without the mouthpiece: The goal is a sound like that of a bee or large fly. Syllables are most effective aids. I use the word DIM or EM to set the lips. Saying it several times, vigorously or even angrily, gives a good feeling for "set" of lip corners and placement of teeth (about the thickness of a tongue-tip apart) and lips.
Try it again, blowing air against closed lips for a feeling of com- compression of the air stream (later known as "support"). Exhale, three or more times, to gain a sensation of firm lip-set and air stream compression.
For a vibration: moisten lips, set, and let a sudden puff (NO TONGUE, please) of air out. Probably, no buzz will occur. Doing it more vigorously, with a teacher demonstration, will eventually produce the desired result. It may help to place two fingers in a "V" over the lips to restrain them or to place the mouthpiece on them and try buzzing. The syllable of the air burst is PEH. Combining DIM-PEH (with no observable movement of the lips except the buzz), gives a good basic set.
Setting the Mouthpiece
The low register of the horn requires that a 2/3 to 1/3 ratio of upper to lower lip within the mouthpiece be used. I place the mouthpiece on the student's embouchure myself and ask them to hold it in place with thumb and forefinger (so I can see). Remove and reset a dozen times; then introduce the buzz. The student's ability to find this "groove" is most important and making a new one for a student who has habitually used too little upper lip is major undertaking...best to find the "groove" from the outset.
No details about right or left hand will be given here except to say that the only way a small player can get a good downward angle to the mouthpiece is to be sure that the left hand pulls the horn leadpipe downward and straight toward the music stand. Most likely the student will need to turn the lower body, or at least the right leg, somewhat to the right.
The Lower Jaw
Once a buzz is consistent (without the horn but with the mouthpiece), it is a good practice to see that the student flexes the lower jaw (I ask them to "chew.") both silently and with a buzz. The effect is rather siren-like. for smooth slurs, consistent quality of tone, and ease of response in range extremes, it is necessary to have lower jaw flexibility. Once again, syllables are useful. As the jaw move downward, high register "ee" becomes middle register "oh" and low register "aw." The lower jay pivots up and inward or down and outward. For students who can whistle, the setting of tongue and jaw is much the same in horn playing as whistling. I often ask students to try whistling intervals of fourths and fifths to get the "ee-oh-aw" oral cavity shapes and sizes.
The only alternative to a flexible jaw is "stretch and pucker." Those are difficult concepts to change later; establishing jaw movement from the outset is worth the effort. The other way encourages pressure. The result is uncertain response and inconsistent tone quality from register to register.
The embouchure is frequently disturbed by the tongue. Hornists use TU and DU or often TDU for a marcato. Generally, it is a broader tongue than one uses on trumpet. It's THU that interferes with both lips and teeth, resulting in a nastiness not appropriate in horn playing.