You know how you drink water from a water fountain? that is how you do it. To slurp up water from the water fountain, you are actually inhaling and at the same time filling your mouth. You have leaned to inhale slowly and in a controlled manner and hold water in your mouth and somehow put dry air in your lungs (before you swallow).
Using a snorkel is not much different... you need to start the inhalation slowly and if you feel, hear, sense, water gurgling in the snorkel then you must use your mouth as a splash guard and inhale very slowly and gently, so that you can get enouh air to blast out the snorkel.
To do this, you must be calm and not sucking air super hard.. inhale slowly and carefully and exhale as hard as you want.. 3 yr olds learn it, it just takes preactice.