How to get cat to lose weight?

Our cat was underweight when we got her from shelter. I have tried hard to follow bag directions on feeding. I found she was "crying" and running at night. Her cup of food was empty. So I thought she was hungry. Now she is getting overweight and I recognize it. I have stopped the night feedings only once a day but how do you get them to lose? I want her to be healthy. Does low fat food help?