Find something with a really strong smell usually a food smell, that the dog will sniff, you can use a dog treat that he does not usually get. Hold it in your closed fist but open a little hole so the smell is released. Have a friend help you by coming to the door. Right as you tell the person to knock hold the smell up to your dog's nose. When he sniffs instead of barking give him a treat from the other hand. Then have him sniff it again and treat. If he stays quiet give another treat. Then have the person knock again and repeat. Then again and this time have him sniff a little longer before he gets the treat. If you want you can let him have two or three barks before you introduce the smell so that he learns a few is ok but no more (if you want him to alert you to people at the door, plus he can still get out his urge to bark).
You can try several things instead, it depends how locked onto the door she is. Some dogs ill pay attention to nothing when the door bell rings while others can be distracted. You can practice by having someone knock on the door and you throwing a toy so she chases that instead of barking. you could whistle to get her to stop and look over at you (then give a treat when she does).
You could also do the ignoring when you and the guest basically just wait there and as soon as the barking stops you give her a treat and attention. then stop if he barks again.
But as Dekka said it's important to find out why he barks he could bark out of fear, for attention, to say "get the he!! out of my house" or just to say "hey mom look who's here!". If he is doing it out of fear or protection then you might have more to work on. If it's for attention or just alerting you then it is easier. If it's fear obviously getting attention when he stops is not going to help he'll just be more frightened of the guest coming to pet him.
Muzzles are ok in theory but it does not teach the dog anything it just physically enables them from barking (actually I don't know if they even do that) plus the last thing you want is the dog to be afraid of muzzles, if he ever has to wear one at the vet or for another reason then you'll have a panicked dog. plus over time when you try to put it on him for barking he may become aggressive.
One things for sure you will need someone to help you by knocking on the door and you'll need to find something that gets the dogs attention so he stops barking.