With Gunnar, the first step I took (with Sherry Woodard of Best Friends guiding me) was to get an Italian basket muzzle.
To get him happy to wear the muzzle, I used cut up cubes of roast beef and put them in the muzzle. The first several times, that's all I did. I didn't attach the muzzle to him. Soon, when he saw the muzzle, he got excited as it meant he was going to get roast beef!
Next step was attaching the muzzle with lots of roast beef - some on the floor which he eats while I'm attaching it and securing it with shoe laces so it doesn't come off. Once he likes wearing the muzzle, he can no longer bite people and I can start working with him.
We had Gunnar tested thyroid problems. Sometimes hypothyroidism can casue aggression in dogs. He was fine. Then we tested for tick diseases. Gunnar tested border line positive for Lyme disease. 3 days into the treatment and we could see how much more relaxed he was.
Gunnar also takes Amitriptyline, which we also saw some positive results in.