What happens next?

How to keep your dog from eating your cat's food
Begin by showing your dog a treat that will fit inside your hand.
The goal with " leave it " is to teach your dog to leave things alone that you don't want him to mess with. To accomplish this, you will train him that he will get something better (a tasty treat) when he " leaves it " as you command.
Show your dog the treat and let him see what you have. Make sure you have a treat ready for him in your other hand, but don't let him see this one.