I suggest that you start by the general principles and facts about overweight. A good place to start is the diet. Are overfeeding it or feeding it on fatty foods? These should be substituted with healthy fats like say, avocadoes which have a healthy fat. What about exercise? This can be achieved by bringing in a companion preferably a kitten so as to be involved in play. You can also reduce the size of the meals while increasing fluids like soups and milk.
The best place to start is with the basics of the predisposing factors for overweight. It could be that you are feeding the poor thing on to much monosaturated fats? The best alternative is avocadoes. You could also bring in a companion so that she can benefit by exercise accruing from play. How about cutting her meal size and frequency accompanied by an increase in healthy foods like soups and milk? And if you have some extra cash to spare, you can take her for liposuction. Hope this helps.