You can lure your dog with a treat. Put some peanut butter or his favourite treat inside the muzzle. Your dog will come close and start licking the muzzle, let him lick all the treat. Do not try to secure the buckle at this point. If you do buckle it up and try to seize him, he will develop trust issues. Instead, you should take things slowly, even if it takes more than two or three days or even a week. You will automatically know when your dog is comfortable around the muzzle. At that time, you can try securing it for 15seconds initially. No doubt, you require a lot of patience here. Or you can consult an expert trainer as an alternative.