[She laughs. Has he really never made a sand castle before? What do angels do in their free time?]
Here. [Gently, she takes the bucket from him.] First, you pile the sand into the bucket. You have to make sure it's packed in, otherwise it'll just fall apart when you turn it upside down.
[She does so, pressing down on the sand in the bucket before flipping the bucket over in one smooth motion.] Then you tap it carefully to dislodge the sand inside a little, lift up the bucket, and you have part of your castle!
[liiiiift and voila! She passes the bucket back to Castiel.]