Gain experience using the snorkel. Begin by laying flat on your stomach, just floating and placing your face in the water at about a 45 degree angle. Bite gently on the mouthpiece, allowing your lips to seal around it and hold the snorkel in place. Test the snorkel by taking regular slow breaths through the tube. Breath slowly, deeply and cautiously through your snorkel. No need to panic you can always lift your head above water if you want. Just relax and become aware of your breaths. The sound of your breathing through the snorkel barrel should become quite noticeable.
Snorkel Clearing technique.
While holding your breath, place your head below the water so that your snorkel is immersed below the surface. You should feel that water is entering the snorkel barrel. Next surface your head without lifting it out of the water and quickly exhale forcefully and sharply into the snorkel. This blast method of snorkel clearing will remove nearly all the water from your snorkel. Any small amount of water left can be removed by a second forceful blast. Keep in mind that water can also enter the snorkel even if you don't immerse you head underwater. Sometimes either due to surf conditions or excessive splashing, some water may enter. So one thing to keep in mind is to breath cautiously. By repeating the blast method you should clear any water that enters into the snorkel.
Master airway control.
Sometimes, although you may have some water left in the snorkel, if you inhale slowly and use airway control, you can breath the air past the water until you have enough air for another blast.
The idea is to keep your fin stroke movements smooth, slow and relaxed. Keep your arms at your sides in order to reduce drag, slow your kick and lengthen the stroke. Your legs should be extended so that the fins are pointed behind you. With your knees slightly bend, kick slowly and powerfully from the hip to make use of your thigh muscles. Avoid kicking with your knees, as this will only waste your energy. The right snorkeling technique is for the power portion of the stroke to be downwards. Kick farther down and less upwards whilst arching your back upward to force your legs downward.
Swim at a steady comfortable pace to conserve your energy
Streamline yourself as much as possible. Keep your arms at your side
Breathing cautiously through your snorkel, to avoid choking on water that may enter.
Keep your fins below the surface when kicking.
Kick slowly and powerfully from the hip. Avoid kicking with your knees.
Use the downward kick stroke more than upward.