looking thru other peoples tutorials i find these 2 to be the most simple and relatively painless
1: Night-Night is easy and clean - u can search SS for the detailed tutorials.
2: Shallow Water Drowning / Hypoxic Blackout: Not discussed often in SS, I've seen in mentioned one or two times only
Just search it up lots of guides on how to prevent it, which means lots of guides on how to induce it.
tldr; Occurs when u dont have enough CO2 to trigger a breathing response, and u just drown without realising
Heres an academic source:
https://www.ncbi.nlm.nih.gov/books/NBK554620/
Don't think I can explain very well
Can be completely painless/even a little euphoric just before blackout, but can also induce a reflex to breath sometimes leading to water entering the lungs which can be very painful and such (based from the article and other sources on google)
I have no experience though and theres not too much info / past experience on it from other people either
some forms of hanging could also be quite fast and painless but u do need some more prep and an appropriate environment