1) The food to be measured is put into a device called a bomb calorimeter. The calorimeter contains a sealed chamber inside another sealed chamber. The food goes in the interior chamber, then water is put in the exterior chamber surrounding the chamber with food.
2) A device is set up to cause a spark that ignites the food in the interior chamber and then a thermometer is set up to measure the change in the temperature of the water in the exterior chamber. The food is burned and then the change in water temperature is measured. (Sidenote: the ash left after all the food is burnt is where the inorganic minerals in that food are found--minerals like iron, zinc, copper, and selenium cannot be destroyed by heat and they do not provide energy).
3) Mathematical equations are figured to determine the energy released (remember that calories are a measure of energy.) For every 1 gram of water that is raised 1 degree Celcius, you have 1 calorie. For every 1,000 calories you get 1 kCal (or the Calories you are used to reading on food labels).
I did my first calorimetry experiment in my chemistry class last week. We burned Cheetos to determine how much heat energy they released so we could see how many Calories they contain. According to the food package, Cheetos contain about 5.71 Cals/gram. In my study, I found that Cheetos contain 4.88 Cals/gram. Now, before you start cheering and rush out to buy some cheesy puffs, you have to figure in the amount of heat lost to the environment in my study (which you would not experience with a bomb calorimeter). Also, my study determined that burnt Cheetos do not smell very good.