Sit up straight at the edge of your chair. Relax your shoulders so that they are positioned slightly back and not crouched over the instrument.
Press your lips together and use the muscles of your cheeks to create additional tension in your lips. If you are having trouble with this, pretend like you are sucking through a straw. The tension created is very similar to the correct embouchure for playing a brass instrument.
Hold your lips together securely and make a buzzing sound with your lips. Keep your lips even and don;'t let the top lip overlap the bottom lip. Continue practicing until you can buzz with your lips.
Press your mouthpiece firmly but gently against your mouth. Attempt to buzz through the mouthpiece using the same process as buzzing without a mouthpiece. When you breathe in, push air down into your diaphragm. When you breathe out, use the muscles in your stomach to expel the air.
Place the mouthpiece in the brass instrument by slightly twisting it. Buzz through the mouthpiece to create a sound. Play long tones to help develop your tone and the strength in the embouchure.