Learning to circular breathe when playing the trumpet can be a difficult thing. It’s all about breathing in through the nose and breathing out through the mouth at the same time. Sounds hard, right? Well, it does take time to learn, but one of the best ways to learn this technique is to start slow. Some recommend attempting to push the air out of your cheeks with your hands to start - not using your lungs at all.
However, it is not recommended that you go gungho and try this on your mouthpiece the first time. It’s suggested you start with a straw in some water. Blow bubbles! As you work on switching between using your cheeks and using your lungs to blow the air out of your mouth, the bubbles will keep going. Keep going back and forth until the bubbles are continuous through the switches.