Quick Answer:  How you introduce a horse muzzle is important.  If you are rushed you could make your horse for ever wary. So number 1;  Take your time, do it slowly and give the horse time to sniff. 2. Make the experience a good one, give treats inside the muzzle
3.  Give treats up through the grazing slot.

All these things set the scene for trouble free putting on of the muzzle and are well worth going slowly with.

A big tip:  Once the horse has had a good look and sniff, smear some honey or Lickit inside the floor of the muzzle.  Do this when standing on short to medium length grass and as the horse presses down to lick, some grass will pop through and will give him the idea.

Check and double check the fit and be especially certain that he can drink with it on.  Every day check for rubs.

The LiteBite horse muzzle is soft, strong and doesn't rub, can't eat out of the sides either so well worth a go.  Muzzles are a great protection from laminitis too.