Either drill holes in the eyes of the coconut or use a knife to poke a hole in the eyes of the coconut. Drain the coconut water and set aside.
Take one coconut and find the seam that goes around it. Cover with a kitchen towel and hold still. Split the coconut open along the seam using a hammer. Keep turning the coconut while hitting it with a hammer along the seam until you go all the way around the coconut.
Once the coconut is open, scoop out the coconut meat and set aside. Repeat for all coconuts.
Rinse the coconut meat to remove all the brown outer pieces. In a blender, combine the coconut meat and coconut water. The coconut water volume can vary, so add boiled, filtered water to the blender as needed. I find a ratio of about 1 cup of coconut meat to 2 cups of coconut water or filtered water works best.
Blend coconut meat and water for about 2-3 minutes. Pour mixture through a sieve or nut milk bag to separate liquid from coconut pieces. Store coconut milk in the refrigerator to use as needed.