Exciting is signed exactly like excited: open both hands with your middle fingers forward a little more than the rest. Tap your fingers around the center of your chest repeatedly, each hand taking turns making upward circular motions as they tap your chest.
Equally important as doing the sign correctly is acting excited as you do the sign. For your baby to understand the meaning of the word, they need to see you being excited.
With all emotion signs, the key to teaching the sign is really exaggerating the emotion while you do the sign so that your baby internalizes the concept. For example, when getting dressed to spend your weekend at the carnival or fair, say, "We are going to have an exciting time!" using a higher pitched voice and bouncing up and down.
Concepts like 'exciting' are tricky to teach using the flash cards. You really want to emphasize your excitement when you are showing the flash card, saying the word, and making the sign.