First is positioning your mouth. I play on the side of my mouth because I feel it creates a better drone with the air being focused closer to the side of the didge. Many players play straight on, which also creates a strong drone. Whatever feels comfortable for you.
Buzz your lips while blowing out. Many try to blow too hard but agaves don’t need much to produce a drone. The tighter the buzz the better the drone. You will get better and better at tightening your lips with time as your muscles get used to the new exercise.
Any sound that you can make will exit the didgeridoo in a cool way. I love hearing people when they first start playing because they create vocals that I would never think of. That is one of the beautiful things about the didge. Bird sounds, such as that of the kookaburra are popular and so are dingo bark sounds. I rarely use these but rather try to create unique sounds or sing along with the drone. Nothing sounds bad so experiment with a variety of sounds.
I use my tongue probably the most when playing. Tapping your tongue on your teeth can create a nice drum sound. Rolling your tongue (as if speaking a double r in Spanish) sounds great and you can add vocals at the same time for a cool effect. Moving your tongue back and forth at different speeds is also a cool effect.
You can pop your cheeks for an explosive sound. Alternate back and forth between the left and right cheeks to create movement. The tighter your cheeks the sharper the drone, whereas with cheeks puffed out the sound is a little flatter. You can make the sound travel up and down the didge alternating between keeping your cheeks in tight to puffing them out.
Toots are a higher pitch sound you can create to add some variation to the drone. I don’t use them as much, (partly because I am not that good at them) but they can add texture to rhythms are a great sound to add to your repertoire. The sound is created like playing a trumpet. So instead of buzzing your lips this would be a tight lipped approach. Through concentration of air you can do several overtone notes. I can get two sometimes three but some players can reach more. Thinner necks on didgeridoos can make it easier to reach these overtones.
I think most people try to do circular breathing too soon. If you have a strong, tight drone, circular breathing will be much easier. So once you feel confident with your drone and some sounds, then try circular breathing. At first this can be really frustrating and seem almost impossible to do. The good thing is that on agaves you need less breath so circular breathing is easier.
The basic concept is to push out air with your cheeks while taking a breath in through your nose and then using this air to continue blowing out. Your lips must continue buzzing during the exchange. Most people will think too much while trying this and once you start thinking, the timing will be off.
There are many exercises to practice circular breathing. One is to use a straw in water so you can see the bubbles you’re producing and see if they form a continuous stream. Another is to fill your cheeks with water and push out the water with you cheek muscles while breathing in through your nose. I would do this in the shower where it is obviously easier and you won’t get water everywhere. This exercise helped me the most because I got used to pushing out while breathing in and the pressure with the water can mimic the pressure when playing the didge.
There are numerous websites which explain this process in great detail. It is a rhythm in itself that comes with practice. Once you learn it and can do it you will realize it is not as difficult as it seems. Once circular breathing is learned the possibilities of what you can do with this instrument are endless.