Start out with a general signs for insects, like bug for younger children. Then as your baby gets more advanced, you can move on to the more specific signs like butterfly. Many babies are fascinated by butterflies, and learning the sign adds to their enjoyment.
Butterfly and other insects are best introduced once your baby is old enough to go on discovery walks in the garden. You can also discuss the concept of a butterfly by showing your baby videos of butterflies and pointing to the image on the flash card.