I recommend starting the gauze method before going to toothbrush cooperative pet gets comfortable with the gauze and your finger in his / her mouth, then try a toothbrush & toothpaste method.
Use a dog toothbrush and toothpaste made for dogs. ( you can use baking soda paste instead. ) If you brush dog's teeth often, you don't need toothpaste, use just a toothbrush dipped in the water.
Stand behind the dog when you brush teeth on a large size dog. Don't attempt it from the front.
It's fairly easy to brush the teeth of large size dog if it is co-operative.
Hold a toothbrush 45 degrees to the teeth. Move a brush as if drawing a small oval shape.
Do it lightly by massaging the gum line at the same time.
The places collect tartar most are the base of canine teeth, and the side of the rear teeth.
Mouth washing to feel better.
When you have acquired more experience, you can brush inside of teeth row and deep back end of teeth too. You can even give him / her dental flossing if it is necessary.
Ask me if you can't figure out how to do it when you visit my shop for your dog's next grooming
Nylon bones (dental bones )you see at pet store seem effective to prevent tartar, but you have to introduce it to your dog and play with it at puppy age. Many adult dogs are not interested in biting those bone unless they were given them at pups.
Do the exercise and teeth brushing at your own risk