Sorry, good luck with that.
Once you have spent some time on this forum, you will know that what you are asking for, does not exist.
NOTHING guarantees a 100% success rate.
Even blowing your brains out with a sawed off shotgun using 00 buckshot, is only 99% guaranteed.
That method fast, but not peaceful, and the gun could be hard to get, and will cost some money.
The most peaceful method with a fairly high rate of success is Nembutal, aka "N".
It is hard to get, and expensive: about $600 USD for enough to guarantee to kill you.
Hanging is cheap, with stuff readily available, but success rate is not so great.
And then there is this little thing called Survival Instinct, aka "SI".
SI is the human body's will to survive, through both voluntary and involuntary actions.
When you ingest something that will kill you, your body will usually puke it up, so you will not die.
That is an example of an involuntary SI.
There are ways to suppress this action with drugs called antimetics.
If you want to "catch the bus" aka ctb, successfully, you will need to examine the methods available on this site.
Decide which one works best for your situation.
Study the method carefully to find out things to avoid.
Acquire everything you need to carry it out, and plan carefully in carrying it out.
That is the best way to guarantee success.
Attempts which are carried out on the spur of the moment are almost never successful.
And, they can fuck up your life, to prevent you from another successful attempt.