LOL how did you not remember Littlefinger literally straight up tells Sansa how it goes down after he gets her on the shipi have to go back and look, but i think its implied that littlefinger and lady olenna did that together. at least thats what i thought. ima check that out today tho cause i aint so sure.
EDIT: i looked on the game of thrones app which is the authority on the story. this is what it says for the queen of thorn's entry
so it looks like yea, lady olenna and littlefinger planned that shyt together. she gave the hairnet to littlefinger who gave it to dontos and during the wedding she grabbed the poison from the hairnet while adjusting sansa's hair.