To sign dining room, first make the sign for eat by bringing your dominant hand to your mouth with all fingers together, and complete the sign using the sign for room, which is made by making a square with both hands directly in front of you.
We use dining room to teach children locations around the house. You could ask a little one to help you take something to the dining room.
The sign for dining room is best for a toddler who is hard at work in vocabulary expansion. Teach it in association to meals.