First, dig a large moat, piling the sand in a tall mountain. Make sure the moat is connected to a vast source of water. Next, mound buckets of wet sand strategically on the mountain. Shape towers and stairways, turrets and walls. Drizzle even wetter sand hither and yon for a Gaudi-esque effect.
Take a break. Go for a long walk to the point. Have a picnic. Check the castle hours later when the tide comes in. Watch closely as each wave makes its way up the sides of the castle, making its own architectural design as it recedes. "Sand Castle" is 38" x 50," a watercolor on paper painted in 1987.