This is not a competition exercise - this is for when you are going for a walk, and realize that your dog is making a beeline for something truly disgusting. You tell him "Leave it!" and he does so. Also works for telling your dog to get away from something else such as a plate of food on the table, or for when you drop something on the ground you really don't want him to have. I even know a a dog that will respond to "Leave it" when his owner wants him to move away from another dog! This is an essential behavior for any dogs who will be doing therapy work.
Okay - get yourself ready. Have a bunch of little treats - some really tasty ones & some so-so ones (I'll use hot dog slices & Cheerios as an example).
Have a couple of Cheerios in one hand which will be the "Leave it" hand (to begin with, anyway) and a hot dog slice in your other hand (your "Get it" hand). Hold out your first hand, open to show the Cheerios. Your dog will, of course, start to reach for them. Say "Leave it!" and close your hand. However, keep the hand down at his level - don't yank it away, just close it into a soft fist. He will probably lick & nibble at your hand, trying to get the Cheerios. When he gives up & pulls his head back you need to immediately C&T, giving the much tastier hot dog from your other hand.
Keep doing this until he is no longer trying for the treat from your Leave it hand. Then... switch hands! Expect to him to about start over at first, but then quickly figure out what is going on. (Ahhh... it's not which hand it's in, it's what she's saying first! Eureka!")
Now, see if he's really getting it. Show the hand with the treat, and this time leave the hand open as you present it and say "Leave It." The odds are quite high that he will try for the treat, so be prepared to shut your hand quickly! Then just wait for him to pull back as before. Eventually you will be able to leave your hand open and your dog won't even try for it. Smart puppy!
At this point, you want to start requiring that your dog not only "leave it," but look up at you before you say "Yes!" and "Get it." To do that, just do what you have been, but after your dog moves away from the leave it hand, just wait.... until he looks up at you, then immediately say "Yes!" and "Get it." If he takes too long to look up, then say his name to get his attention, and reward that.
Next progression would be to set a treat onto the floor and say "Leave it." Reward with a jackpot if he does! Be prepared to step on it (use a dry treat here so it doesn't get smooshed!) to cover it if he doesn't. In that case, just try again. When that is going well, actually drop a treat on the ground, at first just from a few inches off of the ground, then gradually higher.
Note - Try to usually have much better treat in your "get it" hand than what you are making him "leave it." However, make sure to practice this with some pretty high level treats as the Leave it treats as well! When the time comes when you are out in the park & he discovers a rotten frog carcass & starts drooling, you want him to respond to your "Leave it" command. He'll be amazed, thinking you actually have a treat better than rotten frog! Of course you won't (I hope!), but by then it'll be too late for your dog - you'll be past where the frog was. He'll be disappointed, but will survive. And certainly, you would lavish affection on him at that point, and give up any treats you might have on you.