This is a great video Melodica-Me. It’s exactly how I circular breath. Very simple – create a pocket of air in your mouth by puffing out your cheeks. Expel that short burst of air into the melodica to create a note of about one second. At the same time, take a sharp in breath through the nose. By the end of the short note, your lungs will be full of air for you to continue blowing – this time from the lungs.
I learnt circular breathing on a clavietta. It was completely airtight, so not too difficult. The Vibrandoneon also worked fine as long as it was on the single reed setting. The P37D is a great instrument for circular breathing, as it takes so little air to make a note sound.