You've come to the right place.
First, know that cats need to lose weight slowly. No more than 1 pound per month. Unless you have a scale you can use, that makes it very difficult to measure progress!
I used the following method to get Poppy to lose weight. She WAS 25 pounds; I don't yet know how much she's lost. Sophie, who was only 11 pounds, lost 2 pounds since last year. I'm so thrilled! So I know this works.
From my blog:
In a nutshell, most cats are fat because they've been fed too much (free-feeding or indulging them) and because they're fed dry food which is NOT appropriate food for them due to the grain/carb/sugar content. Cats are obligate carnivores who need to eat a high meat-based protein diet, which is also high in fats and low in carbs.
Many vets, who are sadly ignorant on the topic of proper cat nutrition, will recommend prescription diet foods (usually dry). Cats are expected to lose weight on a starvation diet of 1/4 or 1/3 cup of food per day. Everyone is miserable in these situations.
The solution is to switch the fat cat (and any others in the household as well) to a good quality canned food. Because these foods are species appropriate, the cat will lose weight at a safe, slow pace (no more than 1 pound per month). Switching foods needs to occur slowly, and if you're trying to get a kibble junkie to eat canned food, that will happen naturally.
That's it. Easy as pie. Not only will fat cats lose weight, but it and any others in the home will reap many benefits of eating a speciies appropriate diet.
See the links below for detailed information on this topic.