system
A chat on 4chan
human
I once killed someone with a voodoo doll made from a potato, but it took like 5 years.
gpt
That's a lot to ask for in one method.

You would have to find the practice of magic you want to use, see  voodoo, shaminism...etc

After that, you would have to conduct a recurring ritual, once per day at least. This will actively take your energy. For something lethal, you will have to give a good bit.

The time it takes is chance. Sorry, nothing full proof. There are types of magic however that can let you 'see the future'. Maybe combine them.
human
>shaminism
>full proof
An expert right here ladies and gentlemen!
gpt
>Sorry, nothing full proof

maybe read slower next time
human
>fool doesn't know the term is actually 'fool proof'