Prior to starting an overweight cat on a cat weight loss
diet, a physical examination by a veterinarian is a must. Your typical domestic short hair should weight about 8 to 12 pounds with males generally a few pounds heavier than female cats. The ideal body weight will be dependent on the breed of cat and age (cats become more sedentary as they get older). 
If the physical exam including blood workup is normal, then you can begin a gradual cat weight loss program. A cat weight loss program for an obese cat should include a reduction in food, switching to a better cat food with fewer carbohydrates, stopping free feeding and an increase in the amount of exercise.
If you are free feeding (leaving food available 24 hours a day), that is the first thing you need to stop. Free feeding is not natural for animals that had to hunt for their food and can lead to overeating. Feed you cat 2-3 times a day and only leave the food out for a short period of time. Depending on the amount of weight you cat needs to lose, your veterinarian may suggest a weight reduction brand of food or a switch to a more nutritious brand of cat food fed at appropriate levels. If you do switch to a new, better quality brand of food with more animal protein and less carbohydrates or a weight reduction brand of food, mix the new food with the old and slowly decrease the old food. Table scraps should be eliminated and treats, if given, should be nutritious and given sparingly. Fresh water should be available to all cats including those dieting at all times.
You can provide stimulating
toys that will activate the cat’s natural instinct to hunt prey. Climbing and perching posts and hiding food will also increase activity. Climbing
perches with spring loaded toys can initiate jumping games. 
Playing with a wand-type toy or games of chase with your cat will be helpful to both getting your overweight cat some exercise and forming a bond with your cat. The addition of another cat may stimulate them to play together when you can not be there.
An overweight cat on a diet should be weighed every 3 to 4 weeks to document progress and make sure that your cat is not loosing weight too fast which in itself can be a serious medical problem, hepatic lipidosis. Hepatic lipidosis is accumulation of fat in the liver that occurs following a loss of appetite or a diet. Cats should lose only about 1% to 1.5% of their weight a week. It may take up to a year for a severely overweight cat to reach its ideal body condition.
Medical consequences of an overweight cat include diabetes, hepatic lipidosis, arthritis, impaired immune system, urinary tract infections, and a shortened life span so it is important that your overweight cat be placed on a well thought out cat weight loss program. All members of the family need to follow the cat weight loss program for it to be successful!
