We sign soup by creating a bowl with our non-dominant hand and making a spoon with the index and middle fingers of our dominant hand. We spoon the soup all the way to our mouth. (Note that we sign spoon with a very similar sign, except we do not bring the "spoon" all the way to our mouth.)
We use soup when we introduce our babies to this type of food. Chicken and noodles is a favorite in our house.
Soup is a favorite winter food. Teach it early and consider also teaching the signs for spoon and bowl.