To practice "leave it" with your dog, start with a tasty treat under the toe of your shoe, letting your dog sniff, lick and paw the toe of your shoe, then patiently wait until they get bored and give up. The moment they turn away from your shoe, mark it with a word or clicker and reward your dog with a treat from your open palm. Next, add your cue, making eye contact and saying "Leave it!" firmly when your dog first notices the treat under your toe, and reward them with a tasty treat from your hand when they turn away. You can then put a treat on the floor and hover your toe nearby, immediately asking your dog to "leave it," and reward with a treat from your open palm if they leave it. As you progress, drop a treat from a few inches off the floor, asking your dog to "leave it" as you drop, and reward from your hand if they do. Continue to practice in different scenarios, such as gently rolling a treat or toy behind you between your legs, and asking your dog to leave it right when you release the object, marking and rewarding if your dog leaves it. On regular walks, try using your "leave it" cue when they're on their lead, asking your dog to "leave it" when you see anything interesting on the ground or approaching you, and always reward your dog for good behaviour.