Seriously? Find a small body of water (or pool) and go swimming. Take something heavy with you. Do you stay underwater better when you're holding the weight in your hands, or when you squeeze it between your legs, or when you have it somewhere in your midsection, like in your pockets? Should you maybe use multiple weights attached to more than one spot in your body? How much weight is enough to keep you from floating?
If you want to get good at anything, you need to know what you're doing and practice doing it. Killing yourself is no different, except that you should practice one part at a time so you don't end up seriously injured, or dead before you want to die.