there are two rules. from the time you start planning til the time you go through with it - DO NOT, i repeat, DO NOT let anyone else know.
they WILL interfere and ruin your plan and fuck up your future. think you're at rock bottom and it can't get any worse? think again - how about spending the next three months with a stranger or multiple watching you 24/7 so you can't even have a wank or take a shit in peace, horrid processed rubbish frozen or microwaved 'food', not being allowed outside or even to open a window - the panes are of opaque bulletproof plastic, anyhow - nor listen to music or access the internet. not allowed to go anywhere by yourself, goodbye being able to do anything without answering to anyone else about it, oh and have fun with half your wardrobe gone because they think you're dumb enough to try to strangle yourself with drawstrings or cut yourself with a zipper. no pencil to journal with because you might stab yourself with it. nothing to do except watch the same 3 TV channels all day. if you weren't suicidal before, you will sure as all hell become so during your stay. and you gotta take whatever chemicals they dump on you - any valid concerns or debate is brushed away as your 'illness talking'.
the horror story doesn't end there - when they finally release you from glorified prison, you now have a blot on your record that follows you everywhere for the rest of your life. goodbye, ability to get many stable jobs with pensions, own self defence weapons, and good luck convincing anyone who knew you from before that you're fine now and any potential future employers about the large gap on your resume. and if you're lucky enough to live in the glorious estados unidos, you get to worry about the ridiculously large bill from the ambulance ride you never wanted but that they called on you anyway and now you have to pay for it plus the privilege of your prison stay which insurance didn't cover. why the fuck you're paying for insurance, you don't know, and now with all the career difficulty you'll have a fabulous time trying to rustle up the funds for everything. that is, unless you actually do it right this time. but you can't, because everyone in your life is on high alert and treating you like an infant.
remove any potential obstacles to this. if you think you will panic or get sentimental at the last moment and call someone, or are tempted to text your friends or family goodbye, shut off your phone, factory reset it, let the battery die, and leave it in another room or in your living space if your method involves travel.
and as a second rule - just as important - thoroughly research your method and not rely on 'hope this works'. because it won't, and unless you're in a remote area or have no one in your life who will get suspicious or check up on you - see rule 1, because that most likely WILL happen and you will deeply regret it. okay maybe i'm just projecting at this point.