Cats will always respond to food, so build a routine around going out at feeding time. Spend a few days feeding your cats at the same time each day, and before you put the bowl down, tap it with a spoon so the cats can hear it, and associate the sound with their food. Once they are used to this, leave your door open about half an hour before feeding time, and when it's feeding time, stand at the doorway and tap the dish as usual - you will find your cats are within earshot, and will come in. Do this for a few days, and they will learn feeding time, and return to eat. Don'rt worry if they are a bit late - if your cats find alternative food on their wanderings, they may not come back at the usual time, but they will have learned that your house is home, and their food and beds are there, so that is where they will return. Good luck, and enjoy your new friends.