From John Silvers, 2nd Sq Commander:
Eating a "square meal" meant sitting at attention in your chair with your back the distance of your fist with the thumb extended. Your legs and feet were "at attention" also as the lower part of your leg from the knee down was perpendicular to the floor with your feet pointed directly away from you. Sitting at attention with your face looking forward, the only way you could see your tray of food was to look straight down your nose at your food. You could only cut your food into a bite sized piece each time. When you lifted it to your mouth, you lifted your fork straight up from the table and when you reached the elevation of your mouth, you brought your fork, or any other utensil, straight to your mouth. I believe you could rest your right arm holding the fork with your wrist on the edge of the table, but you could not hold a knife, or any other utensil in your left hand, except to cut your meat, for instance. Many times I would pass up certain foods in the serving line because they would be too difficult to eat. Also, if any First Classman addressed you while you were eating, you had to put both arms staight down the sides of your body and say, "Yes Sir, or Maam" and look at him/her. It was also in your best interest to swallow your food before speaking.
(It was also a good idea to not take too big a bite since it made it easier to swallow and answer quickly. Turk Nuri)