No-one has explained how to do this yet as far as I can see (though it is mentioned in part in a guide for the event), so here goes.
There are several tables available. Pick one and sit on one of the seats.
They work like vehicles and an action bar appears when you sit down with a selection of 5 foods. One of the five is illuminated and you can eat it. A sixth option is to pass that food. (Passing each food gets two further achievements.)
Chairs/foods are: Cranberry, Pie, Sweet Potato, Stuffing and Turkey.
Click on a type of food and a buff icon appears at top right saying you've eaten one portion. Keep eating until you've eaten 5 portions and a Well Fed buff icon appears (a cheese) and you get an hour of that buff.
Change to another seat & eat that food 5 times. Same thing (except your previous buff changes).
Rinse, repeat until you've got 5 types of food, 5 portions of each showing top right.
Ding! Quest complete.
Any editing suggestions feel free.