I managed to get a good sound out of one the first time I tried. The guy at the booth who was selling them told me I was about as good as most younger children who tried. I believe the reason is that playing one of these things requires a lack of concern about looking or sounding ridiculous.

This is what I do. (No, I'm not an expert - I just play one on the internet. I CAN make appropriate noises.)

Mash face onto waxed end of digerido. The seal is important.

Completely relax your mouth and make the biggest raspberry noise you can imagine. Not one of those wimpy little fake fart noises. The lips have got to flap. If you can make a horse-snort sound, it's a good start.

I can't manage circular breathing. I believe the technique is similar to the one employed in blowing bubbles with chewing gum. (You have air trapped in your mouth going out, and can breathe in through your nose at the same time.) I can manage the bubble-blowing technique, but I haven't been able to keep the mashing and lip-flapping going while doing it.