Before anything else, you'll need to remove the coconut water from the fruit. To do this there are two main methods - one that keeps the shell intact (for when you're wanting to make coconut bowls with no holes) and one that doesn't.
If you don't need the shell intact, simply hold the coconut in place (you can also place a kitchen towel beneath it, to help keep it steady) to begin with. At the top of the coconut, there are usually 'eyes', where the shell is thinnest. The aim is to poke a hole in the eyes and drain the liquid from it. You can use a screwdriver or large, clean nail and a hammer, to drive it into the shell.
Once you have holes then simply drain the coconut water into a glass. It's good to do this through a sieve, cheesecloth, or nut milk bag, so there are no bits in the water. You can then drink this as is or add it to smoothies and other drinks/recipes.
Once drained it's time to open the coconut using one of the methods outlined below.
The Bang Method:
Put the drained coconut in a bag and bash it against a hard wall (outside of your building preferably) a few times until the shell cracks enough to pull open.
If you want to go completely tool-free then you can simply bang the shell against a hard edge enough to get your first cracks and quickly drain the water into a bowl before continuing to crack it open.
The Hammer Method:
Once again, this method is done after draining the water from the fruit first. Place the drained coconut onto a cloth to stop it from rolling around and then lightly hammer the center of the shell, rotating the fruit as you go. It will crack in half fairly easily this way, although not always evenly.
The Oven Method:
After draining the water, you can use the oven method to crack the shell. Place it on a baking tray in the oven for 15 minutes at 190ºC/375ºF. Removing it from the oven, you'll notice that the shell should have a large crack that's big enough to pry open.
The Saw Method:
This is the one I reserve for when I'm trying to make perfectly even shell bowls. Uneven shells can still be used in a variety of ways: to house candles, hold jewelry, etc. However, if you're wanting coconut bowls you could place food in, then this is the way to go. *
For this method, you just need a small handheld saw. If, for whatever reason, you have some sort of clamp - then go ahead and use it. I just tend to place my coconut on a dish towel to stop it from rolling around and then carefully saw it in half.
BONUS! Freezer Method:
This method isn't so much about actually cracking the shell open, but more to do with making the meat easily removable. Simply place your coconut in the freezer overnight and then use the hammer method above, and the coconut meat should easily detach from the coconut shell, sometimes as a single whole piece!
Notes
Note* One bowl will always have the hole in from where you drained the water unless you choose to saw the coconut over a bowl. I tend to use the side with a hole for hard fruit slices and any foods that won't spill through the hole.