It's best to practice your breath directly on the flute, such as playing a long tone, from the lowest to the highest tone every day. For example, 1 tone, give yourself at least 8 seconds in one breath until your breath is gone. If you practice like this every day, you can keep up with your breathing and your lips won't shake. I think the biggest reason for full lips is that you practice too little.

In addition-take a deep breath! Inhale! Take a deep breath. Be sure to relax all over! Don't shrug or anything. Very, very relaxed. Don't be too nervous with your fingers!