Q. Best way to put on a muzzle on a lab, male dog. He broke my thumb last time we tried putting a muzzle on him.
A. You need to make him believe the muzzle is best thing since sliced bread. No, more referring to it as "the muzzle" it sounds negative and he'll feel that way about it. So from now on it's the "treat basket". Using positive reinforcement and conditioning him with food is a good way to associate the muzzle as being positive and not a panic and struggle, desperately pawing to get it off.
You also need to select the appropriate type of muzzle. Mesh muzzles should ONLY be used for short periods of time, like a vet exam. If you are using it for ANY other reason (walking, training, in the house, and especially exercise) you need a basket style muzzle. It will allow him to breath properly, express himself while training or around other dogs (like growl, bark, showing teeth) and he can still drink with it on.
Most people make the mistake of buying a muzzle, buckle it on and the dog "can get used to it on his own", he will eventually stop clawing at his face to get it off.
That is the wrong way to go about muzzle training, so instead try this:
Sit on the floor with your dog, offer him a treat.
Hold the muzzle in one hand, show him the treat, then place it in the muzzle. Let him take it on his own. If you want to put a command on this like "nose" this is a good time to start. Repeat.
Let him get used to putting his face in the muzzle on his own, taking treats. Once he is comfortable with this start to pet his head, like you could be grabbing the straps to do it up.
Try this several times over the course of the day, always allowing him to move his nose in for the treat, not you scooping it to catch his face.
Next progression, you hold the muzzle on him for just a few seconds. Slide treats through the bars in the bottom and tell him good boy, hold a couple more seconds, good boy and then pull it off. Keep working on building this up SLOWLY.
Next step is to buckle it up, keep feeding treats through the bottom while verbally rewarding, hold for a few seconds, then take it off. Keep repeating this and building it up over time. Remain positive. Great things happen when the treat basket is on. He should be comfortable, relaxed, and not agitated.
If you do this correctly, your dog will be EXCITED to see the muzzle come out. He knows good things happen when it comes out.
I am available for a private session if you need further assistance with this (or the reason causing need for a muzzle).