Again, this is simple enough that even you can understand it. If you consumer more calories than you burn you gain weight. Period. If you consume less you loose weight. The actual quantity of that caloric amount will vary from person to person. Less in smaller individuals, less in women. I can't imagine a way to determine it other than empirically. Then again, determining it is actually quite simple. You decrease calories in until you start loosing weight. Once you do you are below the floor of your needed calories. You boost you caloric intake until your weight stabilizes. At that point calories in equals calories out.
You have to eat less of highly caloric foods, more of low caloric foods. A good example, I'm without my lunch vegetables this holiday season. Just haven't had the time and opportunity line up to get them for the last week. I've been taking serving of pretzels. Boy am I hungry, and I should be. I can't eat as much mass in pretzels as I can in carrots, since pretzels are more densely caloric. So I get hungry.
In a way I have an advantage that can be used by few. I walk to work, so I don't need to carry a wallet. If I've no cash I can't buy food, all I can eat is what I brought. Of course, I have to be very disciplined when I get home (hard to do with lots of yummy holiday stuff around, which would explain my holiday weight gain. It'll come off...).
