The best way to groom a long-haired dog is to start by brushing their coat regularly to prevent mats and tangles. Use a slicker brush or a comb designed for long-haired dogs. It's also important to trim their fur around their eyes, ears, and paws to keep them clean and prevent irritation. Regular baths using a dog-specific shampoo and conditioner can help keep their coat healthy and shiny. Additionally, consider taking your dog to a professional groomer for a trim or a full grooming session to maintain their coat in top condition.