Place a little soft cheese/peanut butter/pate inside the nose of the muzzle. Hold the muzzle in your hand at the level of your pup’s face. As they voluntarily move to you and put their head into the muzzle, say “Good” and give them a treat (squirting more treat into the nose of the muzzle from the outside with their head still in it). Gradually increase the time they have to hold their head in the muzzle before you deliver the treat from 1 second to 2 seconds and so on up to 10. If they pull their head out before you give them the treat, reduce the time for the next repeat before building up again.
Click the muzzle fastener (when the muzzle is not on your pup) and then immediately give them a treat. Repeat.
Once your pup can voluntarily keep their head in the muzzle for 10 seconds, start to touch the straps and place them behind their head while they are licking the treat. Say “Good” and reward as before. Repeat.
Once your pup is comfortable with the straps being touched, you can click them shut, say “Good” and give them a treat, and then immediately undo them and release them. Gradually build up the time that the muzzle is attached before you say “Good”, reward and release, by 1 second at a time.
Once they are comfortable having the muzzle on for ten seconds, you can try taking them out on a walk wearing it.