Ideal way would be somewhere in solitude where I would know 100% no is going to bother me and then whatever poison method such as Sn or N and just relax and think about never having to wake up :).
Practically I'd pick a drowning method such as being unconscious while out "swimming" or the shallow water method, but I tried drowning myself before while drunk and had SI kick in so idk.
I think the classic SN will do, i think it sounds really peaceful and the idea of being able to lie down and relax compared to shooting yourself, drowning, jumping or hanging is really ideal.