To prepare almonds, place in a large bowl and cover with water. Let soak overnight. Drain water and rinse before using.
Place soaked almonds in a powerful blender, such as a Vitamix. Add 3½ cups filtered water and blend on high until completely smooth, about 1 minute.
Slowly pour mixture into a nut milk bag set over a large bowl. Squeeze bottom of bag to extract mylk. (Do not discard almond pulp, freeze to use later for baking or making smoothies.) Transfer mylk to a glass jar and refrigerate. Shake well before drinking. Best served very cold.
NOTE: I highly recommend purchasing a nut milk bag, it makes the process of homemade mylks much easier. A variety of choices can be found online.