My advice is just to practice. Might sound stupid but if you want to be successful, you have to think of suicide as more of a medical procedure and less of a ritual, because that's essentially what suicide is. Suicide by bleeding is essentially just phlebotomy, SWB/hanging is induced obstructive shock, and overdose is medical administration (the only difference is it kills you instead of having a therapeutic effect). The way I practiced is to stand on a mattress or something soft you can fall onto without hurting yourself, and hyperventilating yourself. The way I find best to do it is to breathe as deep as you can in about 0.5 seconds, and then breathe out as much as you can in another 0.5 seconds. The average adult takes one breath every ~6 seconds, so if you do this repeatedly you're essentially breathing 12 times as fast as you normally would. If you breathe that quickly with no physical exertion, your body will replenish oxygen much faster than it needs to and you will be left with very little CO2 in your blood. Breathe at that rate for anywhere from 1 to 15 minutes (how long you have to do it is different for everyone and you just need to figure out what your body is like). Generally, the more fit you are the quicker you can hyperventilate yourself because your lungs will be more efficient at gas exchange (sort of ironic because you probably don't imagine athletes being better at suffocating). Then stand up, put your thumb in your mouth, and blow as hard as you can without letting any air out (this will rapidly drop your BP and further asphyxiate you). If you're so keen, you can also use a pure inert gas to help further asphyxiate you as someone else has mentioned:
That's the basic mechanism of inducing cerebral hypoxia, but it's only really half of the equation because you also need to keep yourself hypoxic after you faint/reach syncope. You don't even really need water technically, because you could probably use a bag tightened secured around your neck and nitrogen (or other) gas to suffocate yourself, but for people with no access to gasses and gas paraphernalia, it might be better to drown yourself (if you do it in a body of water, this can also aid you if you want to make your death look like an accident). You might also just be more comfortable drowning, because creating a gas delivery system is something that could probably be more likely to fail depending on your understanding of gas administration, but that's for you to decide.