The circular Breathing
The circular breathing requires at the beginning a great concentration; but, once you learn it, it becomes automatic and spontaneous. Use the mouth as a sack full of air similar to the bagpipes, so you can blow with high pressure, using the cheeks and the tongue and trying to make short breathings through the nose.

There are many different ways to learn this technique:

Fill your mouth with water and then, using your cheeks and tongue, spray out the water, taking short breathings through the nose. The reason why you use the water is because if the mouth is not perfectly sealed by the tongue when you breath shortly through the nose, the water is automatically swallowed. Therefore it is necessary that the tongue seals efficiently, in order to move forward and backward spraying the water out of the mouth. Repeat this exercise many times to get used to and improve the tongue and cheeks movements.

Or:

Take a drinking straw and twist its ending part in order to create a higher pressure as when you play the didgeridoo. Put the drinking straw in a glass full of water and start blowing, pressing your cheeks and using at the same time the tongue to push out the air left in your mouth. You use a glass with water in order to control through the bubbles the constancy of the breath. Every type of glass may be used, even if a tall glass is more suitable.
To make clear and simple the role and the position of the tongue:
Fill your mouth with air and make short breathings through the nose. Now think over and wonder: "Where is my tongue?" The obvious answer is that the tongue is posed on the palate.

Try to utter any word containing hard sound syllables as "ca-co-ck" (for example: case, company, necklace); through such practice you will note that the upper part of the tongue touching the palate and its sides make an airtight closure of the mouth cavity. This is the right way to work, in order to allow the tongue to move forward, spraying the air (or the water) out of the mouth, while making short breathings through the nose.
The figure shows a cross section of the mouth and of the nose cavity. The illustration represents one of the tongue positions when no circular breathing is required. When this latter is needed, you must position the tongue in order to allow "A" to touch "B" on the palate, using the tongue sides to seal and block the mouth cavity. At this point, move the tongue forward to make "A" move towards "C". Always considering an efficient "Closure" (of the mouth), the air in the mouth cavity must be expelled under pressure, through light leaps vibrations. When the tongue moves forward to expel the air, you have to make short breathings through the nose (the tongue movement in the mouth can be compared to that of a piston running on in a motor).