Some dogs bite while at the veterinarian's office or if they're injured and you're attempting to help them. A dog shouldn't be muzzled if it's vomiting, having trouble breathing or unconscious. Otherwise, there are a few ways to successfully muzzle a dog when absolutely necessary.
Use a commercially made muzzle if one's available. Soft cloth muzzles are the best. These slip over the dog's nose and mouth and have Velcro closures that fasten together behind the ears. Make sure you use one neither too large nor too small for your dog.
Use a strip of cloth or a scarf. Tie the cloth around your dog's snout. Bring the two ends under the ears, and tie them behind its head.
Muzzle your dog using adhesive tape in an absolute emergency. Simply wind the tape around its snout a couple of times.