To sign be careful, start with both hands as a fist while extending your index and middle fingers at 90 degrees to your palm, thumbs resting at the vertex of the index and middle fingers (or in other words, make the 'K' sign in ASL with both hands). Take your dominant hand and tap it on top of your non-dominant hand to make the sign for be careful or just careful.
An alternative sign is to rest one 'K' hand on top of the other 'K' hand, and move both hands in circular motion away from and toward the body, like the wheel of a bicycle.
Be careful is a useful admonition to older toddlers to exercise some caution. This is also a good sign to keep in your repertoire as they get older. Being able to subtly caution a five year-old is particularly useful when you have company and don't want to embarrass them in front of their friends by saying the words be careful out loud.
The be careful sign is useful to help gently warn your child. This is particularly the case as your child gets older, where a public warning may embarrass them or lead to obstinacy. The sign is best introduced through real-life exposure rather than the flash card.