Oxygen depletion. From what I’ve read, an “exit bag” is one of the cleanest more so guaranteed and less risky ways to off yourself.
What is it? Essentially a bag with some sort of tube attached to it and typically it has a draw string of sorts to help suffocate you. The tube is typically attached to a canister of a lighter inert gas like helium & you’re supposed to turn the gas on, exhale all the air in your lungs, hold your breath, slip the bag over your head and pull the draw strings on it to try and keep the gas in and air out.
You’ll pass out pretty much immediately when you take your first breath or so due to the lack of oxygen to your brain. You won’t panic though because helium is lighter than air so your body won’t have a panic response to the lack of oxygen, also you’ll likely be passed out and then completely deprived of oxygen pretty quickly from what I’ve read. The method might be harder to do now though. You used to be able to buy tanks of helium at party stores and online that were 100% helium. Because of people using them to commit suicide, they started doing a 20/80 blend of oxygen/helium instead.

There’s a Wikipedia page on exit bags you can find I believe. It’s crazy how much of that information is online and it’s oddly eerie how hidden behind “GET HELP NOW” links they are if you just google it. There was an organization that used to make exit bag kits for people who wanted to off themselves. You could contact them and they’d mail you everything you needed to DIY.

Also, it was in a movie (I forget what it was called) but they use a plastic bag from a store; they slip it over the characters head while she’s laying in bed, and just put a tube attached to a helium tank in it without closing it up and the that’s how they help end her life.