Place a large hairband over your dog's body, on the area of its body that you would eventually like to clothe. Reward the dog for letting you place the hairband on it, in the form of affection, games or treats. At first, accept one second of leaving the hairband on as a "win," then work toward two seconds, five seconds, and 10 seconds. Do not give your dog affection when it paws the hairband away; this is a surefire way to train the dog to take off its clothing.
Introduce your dog to starter outfits after you have sufficiently trained it with the hairband. These outfits should be light and loose. Don't make your dog submit to wearing a piece of clothing without reassuring it. Be soothing and patient.
Dress up your dog as often as possible to get it used to clothing; dogs learn by repetition and routine. Provide it with positive reinforcement, in the form of affection or treats, whenever you dress the dog.
Introduce new kinds of clothing, such as designer suits and tighter T-shirts, to your dog when it is accustomed to clothing.