Good question.
Here's the golden secret: Start slow. And keep it slow. There are actually different "degrees" of vibrato (wide slow, wide fast, narrow slow, narrow fast...) but for now, it should be extremely wide (too wide?) and extremely slow (too slow.)
There are two steps that I have been taking to learn vibrato, and I will attach a link to the source.
- Rubbing.
To get the movement right, put your instrument under your chin and drop the bow. Put your thumb under the wood and hold the instrument's body, right next to the neck. Then slide your fingers back and forth, forward and backward, on the wood of the instrument. The point here is to only move your wrist. This is hard; it's not really natural. That's why we practice it.
Video:
- Long, slow, high vibrato.
No video for this one, but it's easier to understand. A few steps:
- Set your metronome for 60
- Get up into 5th position
- Using your second finger, play a very slow vibrato - one movement per beat
- Double this to two movements, then four, all the way until eight. Although this may feel slow, this is about the pace that you want to take most vibrato, although slower pieces generally have slower vibrato, and quicker pieces faster vibrato.
Do this last one every day before you play anything else for at least one minute (your hand may be a little sore -- just make sure that you only move your wrist on both of these exercises for now. One problem that I struggle with is trying to do both a wrist vibrato and a full arm vibrato at the same time, and that put my wrist out for a weekput my wrist out for a week.) and you will be amazed at how fast it will start carrying into your other playing.
Here's a good page to look at (you will find that first exercise as well as a plethora of other good ones here): http://www.violinmasterclass.com/en/masterclasses/left-hand/vibrato
Happy viola-ing! Or, uh, viola playing! Or, dang it, whatever it is.