How to slim my fat cat down!?

we have a fat cat named bob who ways about the same he has been that much 4 about a year now, this is cuz we took him to the vet they said that is ok as long as he doesnt gain anymore (or at least TO much more) this can lead to high blood or diabeties so if she gains over 2 pounds in the next couple months take her to the vat and talk to them about it ;)