Head collars such as Haltis or Gentle Leaders are great tools to help you have better control over your dog (shout out to all my Great Dane peeps out there). The concept between the head collar for a dog and a halter for a horse is the same. If you have control over their heads, you can better direct and control the animal. Haltis were absolute lifesavers for me when I had my first Great Dane, Dolly.
However, there is one problem with head collars: dogs hate them. So, how do we use them? First, you have to condition your dog to the head collar. Make it a positive and fun experience instead of fighting your dog. Hold the head collar in one hand, then hold a high value treat with your other hand through the nose band of the head collar to encourage your dog to put his nose near the head collar. Mark and reward when he does put his nose in the nose band, then take the head collar away.
Keep repeating this exercise over a period of time, and as the dog becomes more accepting of the head collar you will start to move the buckles around. Over time, you'll get to the point where you can snap the buckles together. After you snap them together, treat the dog immediately, then take the head collar off. As the dog gets more comfortable wearing it, you can start prolonging the time he wears it for conditioning exercises, and eventually you can start using it for walks. Remember to make training fun, and reach out if you have any questions or problems!