The best way to groom a long-haired cat is to brush them regularly to prevent matting and tangles in their fur. Use a comb or brush specifically designed for long-haired cats, and start grooming them from an early age to get them used to the process. It's also important to trim their fur around their hindquarters to prevent feces from getting stuck in their fur. If your cat is not comfortable with grooming, consider taking them to a professional groomer for help.