Before you crack your coconut you need to check if it is fresh. You can do this by giving it a shake to see if there is water inside; if you can't hear any water it is most likely off, so just discard it.
Push your screwdriver through the softest of the 3 eyes of your coconut. Drain the water out through this hole.
To make it easier to crack the shell, put the coconut in an oven at 150 degrees Celsius for about 15 minutes.
Hold the coconut in your towel and tap firmly with the hammer along the cracks until the whole shell has come off.