In the beginning all you need to do is establish a "frog" which is like the sound you make when you immitate a creeking door. So you find that gentle frog and keep the vibration slow. The idea is that after a while you'll develop more control. You can move that vibration fast or slow. So you want to be able to establish and then sustain the frog tone. Once you get that low subtone, that's your foundation. Then later, once you can sustain it, you add the vowel sound to it. That gives you the rich lows. Eventually when you get the frog you can make it more nasal and get higher overtones. All good things will be built on that foundation if you can establish that control.