Seems like I am a lucky chap
, it has been second nature to me.
One exercise that seems to work is with a closed mouth, jam your tongue against your top front teeth and pull an exaggerated 'grin.' Make sure your mouth is firmly closed, tongue is pressed quite hard and your grin is quite 'tense.' You know you are doing it right (or any of the other techniques) when you hear a 'click,' (or series of clicks) if you hold it you will also notice something like distortion, but not quite (hard to describe).
In several exercises it says to do them with an open mouth, I generally find it much easier to do it with my mouth closed and slightly clenched.
An easy way to test if it is working is to do a manual 'equilise' (out of water) and then use this reflex to clear your ears (should take only one or two attempts).
It isn't actually any of the muscles that these exercises are using that controls this, when you learn to actually control them you can equilise on demand (you just have to be a little careful, on several occasions I have not been paying attention, not realising I was not equilising properly and getting a massive squeeze before being able to equilise manually).
Best of luck,