Open your mouth so that your lips are about 1/2 inch apart. Keep your lips and jaws relaxed.
Press the tip of your tongue against the roof of your mouth just behind the teeth. Slide your tongue backward slightly so that it isn't quite touching your front teeth, and apply pressure to the roof of your mouth with your tongue.
Lift the sides of your tongue so that they touch the insides of your molars, forming a sealed resonating chamber between your tongue and the roof of your mouth.
Lower a tiny portion of your tongue on both sides near the rear of the inside of your molars. This should create a small vent on either side through which you can breathe.
Hum with a "huuuuhhh" sound as you raise and lower the middle part of your tongue. You should hear variations in your tone.
Experiment with variations in tone by varying your volume as you hum. Create other variations by changing your main sound to "oooo" rather than "huuuhhhh." Vary the shape of your lips as well to further modify your sound.