There is no 100% failproof method tbh.
I was looking into CO in my car, i read the entire megathread over several days and devised a plan, found some items I would need to purchase. I am confident I would achieve my goal...
But in the end the thing I struggle with the most in life Is, well, doing anything at all, and that one requires a lot of effort, so I have resigned to my original trusty method: tie a climbing rope around a tree branch, whack a slipknot over my head, and do a partial-suspension hanging in the garden (partial sus is when your feet or knees are still touching the ground so it's not as painful). Hanging is by no means failproof but there *is* a reason it's the most common suicide method. As long as you have a strong hang point, a strong rope, a solid knot, about an hour to yourself, and a good resolve to continue even if it is uncomfortable, i think its pretty hard to mess up.
I read somewhere in a medical journal that partial hanging are actually super effective and a large proportion of the studied youth cases appeared to be kids just trying it out or experimenting and they actually accidentally followed through. I find this reassuring!
There are megathreads for both hanging and CO, both of which I read, and I appreciate that they are extremely long and repetitive and overloaded with info so I am happy to answer any further question about hanging or CO to the best of my knowledge if you want. Stay safe.