Bucket looks like you are holding the handle of a bucket. Take your open, dominant hand, fingers open and faced down, then lift it a short distance upward, curving your hand into a fist and bobbing it up and down.
Bucket is very similar to the ASL sign for bag, which is signed with the hand already in a fist as it bobs up and down on one's side.
Buckets are a great storage container. After your baby has finished playing with his toys, you can ask him to put toys away in his bucket. Buckets are also good for teach concepts like in and out.
Chances are, your baby will love water and all the games associated with bath time or pool time. Teach the sign contextually by providing your baby a little bucket to use when he or she has access to water. Remember to practice safety and make sure the bucket is small enough to ensure there is no risk of accidental drowning.