First of all, you will want to remove the water before cracking it open. You can do this by using a sharp knife to pierce the eye(one should be soft) using a turning motion.
Flip the coconut upside down to drain the water, depending on how many holes you make will determine how quickly or slowly the water comes out.
Hold the coconut in your hand (hold it lengthways) and use a hammer/chefs knife or cleaver to hit the centre of the coconut and turn it as you repeat the process until you create a fracture line.
The coconut should crack in half where you created the line
If you don't feel confident with doing this method some people wrap the coconut in a towel/bag and hit it with a rolling pin or throw it on the ground - you choose!!
Use a sharp knife or end of a spoon to pry the coconut from the shell (break the coconut into small pieces if it helps)
Peel or scrape off the brown skin with a vegetable peeler or knife. This is optional, only do this if you are using the excess flesh for something else once the milk is made)
Chop the coconut up into small pieces if necessary and then wash it off
Add it to your blender/food processor along with the water.
Pulse until the mixture is somewhat smooth.
Pour the mixture into a muslin/cheesecloth or a fine mesh sieve and squeeze out the excess liquid
Store in a jug/mason jar and refrigerate