A "foolproof way" refers to a method or plan that is very easy to follow and guarantees success, even for someone who may not be very smart or skilled. It means that there is no way to make a mistake or fail if you use that method. Full definition
You already know the most foolproof way of coping with a cat in heat (or one responding to a cat in heat): Get your feline friends spayed or neutered! (pets.webmd.com)
The simplest and perhaps most foolproof way to do it would be to simply follow the monster all the way back to its nest, wherever that may be, and wait for it to fall asleep. (gamecrate.com)
It seems like the only foolproof way for independent studios to keep full ownership at a Big 4 network these days is to have a hot revival, like Roseanne (Carsey - Werner) and Murphy Brown (Warner Bros TV). (deadline.com)