Do not inflate the cheeks too much. Do not force the lungs inhaling too much air. You have to breathe as normally as possible. The most challenging point is adjusting the mouthpiece of the Shofar on your lips, preventing air from escaping from any other point of the mouth, at finding the correct angle. This requires practice.
Again, the steps to effectively blow the Shofar are:
1. Closing the lips.
2. Make the lips vibrate with the letter «P».
3. Place the mouthpiece of the Shofar on the lips.
4. Keep moving and pressing the Shofar on the lips while you blow, until you find the right angle.
THE VOICES OF SHOFAR (qolot)
Each Shofar has a unique voice, like human beings. And each Shofar emits a single musical note. In Rosh HaShana we produce that note in three different ways, which the Sages call “voices” (or “sounds”, qolot) The first “voice” is called teqi’a. Teqi’a is the longest, uninterrupted and simplest sound. Then we have shebarim, dividing the sound of the Shofar into three short voices, interrupting the air that reaches the Shofar by covering the mouthpiece with the tongue. Finally, we have the teru’a , nine very short voices. To produce the sound of teru’a with tap rapidly the mouthpiece of the Shofar with our tongue, approximately nine times.
In this video (between 6-9 seconds) is the most common way to produce the sound of the teru’a