Welcome to BYC!
If you're feeding them regular chicken feed, the easiest thing to do would be to just give them a feeder full of feed. Most chickens will self-regulate and won't eat too much. I'm not sure what makes an overweight chicken, but none of ours have been so far. I suppose it might matter if they free range or not or if they have enough run space...in other words, do they get enough exercise? If they're cooped up in tighter quarters you might want to limit feed if they start to appear overweight. I believe I've read about 1/2 cup of feed per chicken per day.
However, if you do limit feed and start to see feather picking behavior, it's either a sign of too little protein or not enough feed all around. But if they are laying regularly and otherwise seem happy and healthy, keep doing what you're doing.
If they're not allowed to free range (because of intense predator pressure, for example), you might consider adding some fodder (mature sprouts, like wheatgrass) and/or start a mealworm farm to make up for some of the nutrition they would otherwise get from free-ranging. Or plant a chicken garden in their run, covered with chicken fencing, so they can nibble but can't eat the plants to the ground.
