Take it slow:
Take it slow in the beginning as taking your snorkel mask off underwater can disorient most people. Make sure that you try this technique in shallow water first and then move on to deeper areas once you perfect it.
Try to find a nice calm area to try this technique of clearing your snorkel mask underwater. Make sure you are breathing through the regulator.
Gently pull the silicone skirt that sits around the mask slightly away. This will allow water to enter inside your mask. Once the mask gets filled up with water, you can push the silicone skirting back to normal. Make sure that you are still breathing through your mouth with the regulator. It is up to you if you keep your eyes open or closed.
Gently push the plastic part of the mask up against your forehead. This will basically create an angle to help the water seep back out from the bottom of the mask.
Look up or down:
Depending on the type of snorkel mask that you own, you can either look up or if the vents are on the bottom then you can just look downward.
Breathe in from your mouth and exhale using your nose. You will begin to notice that the water is escaping from the bottom of the snorkel mask.
If you notice that there is still water inside your snorkel mask, then repeat the procedure from when you pushed the plastic part of the mask against your forehead. Just keep repeating these last few steps and maintain normal breathing. After a few attempts you will figure out how to quickly and effectively clean your snorkel mask underwater.