Training Your Dog to Wear a Muzzle
The best type of muzzle to purchase is a basket type muzzle, which has holes through which you can feed treats and the dog has enough space to pant. If the muzzle is only going to be worn for 20 minutes and in temperatures under 75 degrees, you can use a "tube-type" muzzle which are found at most pet stores.
Show your dog the muzzle and then feed your dog a treat. Repeat until your dog is excited to see the muzzle appear.
Then put a treat in the muzzle. The "game" is for the dog to reach her nose into the muzzle to get the goodie.
Once your dog is eagerly shoving her nose into the muzzle to get the scrumptious treat simply let go of the muzzle and offer your dog a treat so that she tilts her head up. Have a handful of treats and keep treating. The object of this part of the game is to keep the muzzle on by herself
If your dog lowers her head and shakes the muzzle off, take a short training break while you ignore her. Then begin again. Your dog will very soon learn to shove her nose into the loop for the treat and keep her head tilted up so the muzzle doesn't fall off
When you reach this stage and it could be in a few hours to a few days, you are almost home free! The next step is to fasten the strap and jackpot your dog for allowing this. Practice having her wear the muzzle on for 2-3 minutes and feeding tiny treats or a meal and then taking it off and ignoring her for a few minutes. Do this several times per day. Gradually extend the time she wears the muzzle until she is comfortable wearing if for 30 minutes. Only take the muzzle off when she is calm (always!).
If your dog tries to paw or scratch the muzzle off you can redirect her attention and then offer her a treat or if it isn't too bad, you can ignore it. Be sure to reward her when she stops and is calm. Usually this goes best if the shaping process has been done slowly with lots of treats.
To help your dog make a positive association to the muzzle, put the muzzle on to do things that she enjoys like taking her for a walk!
Here's a great video to help you learn to teach your dog to enjoy wearing a muzzle: