How do I get my cat to lose weight?

I have 3 cats in the house. The oldest is a 7 year old spayed female. The next is a 6 year old neutered male. The youngest is 15 months old and is a spayed female. The youngest is my problem child. Since she was spayed at about 4 months old she has steadily gained weight. She has a large tummy waddle. lol Since she is a smaller breed cat with short legs she has a hard time jumping up on things.I have always "free fed" the older two cats so when she joined the clan I did not change my method of feeding. Now I don't know what to do to help this little girl loose weight. My first thought was t just put food down a couple times a day but that just won't work for the 2 older cats. Anyone have suggestions?