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I will do my best to explain. It’s much easier to show in person.
First you need a source of O2 underwater or be able to hold your breath for a while. A weight helps a lot to keep you anchored to the bottom. Full-face mask also helps, as you are trying to stay as parallel to the surface as possible. Once you have air in your lungs and are anchored to the bottom you are ready to go!
In a current free environment, get your mouth as parallel to the surface as possible. Push your tongue out and push against it with the sides of your mouth. You are trying to make as close to a circle as you can with your tongue and mouth. This will make an eclipse-ish shape. Expel the air from your mouth and lungs as hard as you can. It will feel like if you were trying to set a distance record for hocking a huge snotty loogie.
It takes a lot of practice and tweaking to get it right. I have taught many people, some get it right away others never get the hang of it.
Good luck, Happy Bubbles!