Very very slowly to lose the weight needed. He was 6 when I got him so he'd probably been overweight for sometime. I just fed him the correct amount of food and then made sure he wasn't being fed by any neighbours once he was going outside. I used to tie some coloured feathers to a piece of string and walk around with it attatched to my belt when I was doing my housework, the cats used to chase it so I knew that Smokey was getting some exercise. I also used catnip toys to encourage him to play. It took over a year to get his weight down, I used to sit him on my bathroom scales to weigh him and make sure he was't losing weight too slowly or too fast.
To make sure my neighbours weren't feeding him I went round and explained to them why they couldn't feed him, I also had a dog tag on his collar saying that he was on a special diet and not to feed him, also had my phone number on there too and people would ring to say that he was visiting them and were they allowed to give him a saucer of milk!!! lol