Perhaps the best advice I can give you is to research the Samurai method of Sepukku. It is what you are planning to do, and following such a legendary tradition I believe will certainly give you the best results.
But know that there is no secret magic behind this method: it is all in the mind. Such is suffering, a mere thought, a temporary feeling. But the peace that awaits us in the afterlife, is eternal.
Should you decide to proceed with this method, above all else, this is the time for you to sit down, in silence, and think if this is what you want to do. Fully grasp the decision you are about to take, and be sure you actually want to end it all.
Should you accept this is how you want to go, then I would advice you to research as much as you can on the time given the Sepukku, and the samurai philosophy. Use it to prepare your mind for what is to come. And when the time comes, if you are absolutely certain of your decision, and if your mind is completely clean of any negative thoughts, and focused on the future of eternal peace ahead of you, I promise you, no pain will afflict you, and you will have the peaceful death you deserve.
Should you decide to stay with us, I wish you a long and plentiful life. Should you decide to leave us, I wish you nothing, for I know you shall find peace.