Hi Philipp,

Circular breathing takes a LOT of practice.
The idea is to breath out about half of your air in your lungs and then fill your cheaks with air. While pushing out the air in your cheaks with the cheak muscles take a fast breath in through the nose and start blowing again from the lungs. In the beginning make sure you don't blow to loud otherwise you'll run out of air.
First start with developing your cheak muscles. Play a high G note with puffed cheaks and breath in through your nose. Breathing in through your nose should give a "low" breath. Your about to feel a burning sensation in your cheaks meaning that your muscles are growing. Also, in the beginning the pitch of the note will drop because of the lack of tension on the reed.
After a few days of doing this try to breath in while still pushing the air with cheak muscles.
After practising this you'll need to try and shift from cheak to lungs again.
Don't despare, it will probably take you about six months before you'll succeed to do the full circular breathing sequence without losing tone.

Practise, practise and practise every day and you'll get there.

Success,
JB