The best way to train a cat to be vegetarian is by understanding that you and and your cat are not the same species with the same needs.
If you try to force a cat to eat only vegetable matter, it will eventually die.
Cats are not made to be vegetarians, they are carnivores.
Humans, on the other hand, are omnivores, which means we can survive on animal and/or plant matter.
A cat’s teeth and body are designed for meat and the vital nutrients it supplies; the same nutrients are not found in meat substitutes like soy.
Without those essential nutrients, a cat’s organs will fail.
If you really want a vegetarian pet, consider a rabbit or a hamster.