First of all, consult your vet just to rule out any medical conditions and to get a target weight.
Secondly, just like with humans, it comes down to diet and exercise. You can switch to a light indoor formula and LIMIT THE SERVING SIZE. I know it's very hard to do. When my kitty was first losing weight (under the care of her veternarian), she would lay by her food bowl waiting for me to feed her. Sometimes she'd take a nice long nap!!
However, as her meowmy, I'm responsible for her health. Therefore it's my responsibility to monitor her food intake. She didn't starve to death and she actually became more active.
Third, increase play sessions and/or get her a cat tree to climb on for more exercise. Also, you can put her food and water up on a counter or up on the landing of the stairway (bottom of the stairs if you have a basement), so that she'll have a bit more exercise.
It's all about diet and exercise. My kitty's lost about 5 pounds over the past 3 years or so. It was hard at the beginning, but now she's doing great!