The best way to acclimate your dog to a muzzle is by shaping it- minimal words, maximum communication. Not only does free-shaping and shaping build confidence in your dog, but it sets a very clean and concise tone of expected rewards and participation. Shaping the muzzle creates participation from your dog, which is especially important when you have a dog that associates the muzzle with negative things and is put off merely by its presence.
Shaping takes part in approximations. You start with rewarding the minimal interaction between the dog and the muzzle and work your way up to the end behavior.
Start with marking and rewarding them for just looking at the muzzle
As they process why they are being rewarded, they will begin to get closer to the muzzle, eventually venturing to touch it.
As these behaviors progress, you raise the bar of expectation that solicits reward.
For example, if the dog begins to touch the muzzle, you are no longer rewarding just looking at the muzzle - always progressing in the offered behavior.
When your dog understands the concept that placing their snout in the muzzle is what is reducing the reward and it is a consistently offered behavior, like most other behaviors, you want to begin to push for the duration of that position.
Duration of holding their snout in the muzzle will allow the handler to buckle the muzzle without much movement or difficulties.
Some dogs momentarily regress out of frustration, they may paw at the muzzle or bark. Hold your expectations and take the smallest amount of calm duration, progressing the duration as each step is consistently offered.
By the end, you should have a dog that will put its own snout in the muzzle and hold it as you buckle it.
Muzzle acclimation does take time but is what we call cooperative care and it is essential to maintaining a healthy relationship with your dog!