To do the baby sign for long, take your non-dominant hand and stretch out your arm in front of you. Then take your dominant hand, make it into a fist, and just stick your pointer or index finger out. Then run your pointer up your arm, starting from the top of the hand and ending at your forearm or shoulder as you say the word long.
We mainly use long to express time, distance, and size of different objects, locations, or activities. As your baby gets more advanced, you can introduce the added challenge of using size, distance, or length as a descriptor of a variety of objects such as animals, like ant and caterpillar. You can also use it contextually with food, to describe a noodle or other items that are long in appearance, such as a cucumber.
Use the long flash card in conjunction with the short and small flash cards. We use these signs to teach our babies to communicate distance and size.