Use a squished "O" hand as if stuffing a piece of food into your mouth.
(Keep your mouth closed though. Thanks.)
If there are several people and/or a lot of eating going on you might see
EAT done with two hands:
"Last night my family ate popcorn and watched a movie."
The sign for "eat" and the sign for "food" are used pretty much interchangeably. Technically the sign
eat should be a single motion, and the sign for food should be a double motion
(if you consider these signs a noun/verb pair). But what about the concept
of "eating?" I use a double motion to indicate a process is
taking place. How about "pig out?" For that I use both hands. In any case, I wouldn't get uptight about this sign.
If your teacher wants it one way or the other...do it his/her way. If you get
corrected by a Deaf person, just smile and try to remember to sign it that
way around that person.
Sentence: "What is your favorite food?"
= YOUR FAVORITE FOOD WHAT?