Deep is signed a bit like down. First, hold up your non-dominant flat hand vertically, facing one side of your body. Then do the sign for down with your dominant hand right beside your non-dominant palm, simply point your index finger down, and move your hand in a downward motion.
This sign is used to show direction. You can teach your child this sign if you are a family of divers, and you are describing that your spouse goes diving deep down in the ocean. You can also teach this sign to indicate that something else that you are seeing is deep, like a swimming pool, or Santa's big sack of Christmas gifts!
This is a useful flash card taught best in conjunction with other adjectives or modifying words to help your child understand the characteristics of geographical formations (caves, underwater caverns) or more ordinary objects that they encounter in their daily lives (baskets, sacks, etc.).