Not that I condone suicide, but if your going to do it then you wanna do it properly and peacefully.

Buy a nitrogen tank, ideally 2L - 5L. You'll also need to fit a regulator to the tank to control flow of the gas.

Run it through a tube and oxygen mask or modified full face snorkel.

You're gonna want to have a constant flow of nitrogen dispersing into the mask for atleast 6 - 10 mins to ensure death and so you don't wake up with severe brain damage.

Make sure to take a good breath in, breathe it all out, then stick mask on and breath in again.

You won't feel asthough you are suffocating.

You'll just pass out within 30 seconds due to oxygen deprivation.

If you do this in a room, open a window so that you ensure the safety of anyone that finds your corpse, otherwise you'll possibly gas them unconscious too.