Try to avoid toothpastes with baking soda, detergents, or salt sometimes found in human pastes. Fluoride may be incorporated to help control bacteria. Rather than placing the paste on top of the brush try to place it between the bristles. This allows the paste to spend the most time next to the teeth. Step three is to get the brush with paste into your dog's mouth and all the teeth brushed.
Most pets accept brushing if they are approached in a gentle manner.
If you can start when they are young, it's quite easy, but even older pets will accept the process. Start slowly, you can use a washcloth or piece of gauze to wipe the teeth, front and back in the same manner you will eventually be using the toothbrush. Do this twice daily for about two weeks and your dog or cat should be familiar with the approach. Then take the pet toothbrush, soak it in warm water and start brushing daily for several days. When your pet accepts this brushing, add the toothpaste.