Thank you for submitting your question regarding Snowy. If she has smelly breath, then she likely has an infection in her mouth causing dental disease. If so, she will need to go under anesthesia to have her teeth cleaned by her veterinarian. Afterwards brushing will be less painful to her.
Start slowly and use a toothpaste formulated for animals. Start by offering the paste on your finger as a treat. Once she likes it, then slowly start brushing. You can use a small toothbrush or a finger brush. I hope this information helps!