The bunny/rabbit sign looks like the long ears of a rabbit, twitching. Make both hands into fists and place them on top of your head, with middle and index fingers extended to make the rabbit ears. Bend and extend the middle and index fingers to the back of your head, as if to make the ears twitch.
An alternative ASL sign for rabbit or bunny is to just use one hand, your dominant hand, and make the index and middle fingers twitch back as you hold the rest of your fingers in a fist on one side of your head.
Animal signs like bunny/rabbit are a great addition to animal stories or songs. Signs like rabbit are fun and animated, making them particularly entertaining for young children. With animals, we often make the accompanying sounds; for rabbit, you may want to squeak a little, or just wiggle your nose. The same sign is used for both bunny and rabbit.
This is a favorite of many babies. There are plenty of contextual opportunities to teach this sign. Consider getting your baby a toy bunny and combine the sign with images from story books and flash cards.