Making a vibrating sound with your lips is the first step towards learning to play a brass instrument. It is possible to vibrate entire scales with just your lips. The trick is to build the muscles in the face and improve your flexibility. Vibrating without a mouthpiece is much more difficult than vibrating with a mouthpiece. Many musicians cheat by using the pressure from the mouthpiece to create additional tension. This makes learning to vibrate only with your lips a crucial skill since it will help you become a better performer.
Press your top and bottom lip tightly together without letting either one overlap.
Tuck in your cheeks to ensure that you are using all the muscles of your face to hold the tension.
Increase the tension by pretending you are sucking through a straw while maintaining your lip tension. By doing this, you will be using all the muscles in your embouchure.
Vibrate your lips by letting a small amount of air through the center of your lips. Blow quickly and keep the embouchure that you have formed. With a little practice you will be able to vibrate your lips.
Develop your skill further by learning to buzz songs and scales. Just match the pitch by modifying tension in the lips. Tighter lips create higher notes while more relaxed tension creates lower notes.