Moisten the sand with water so that it becomes sticky. Pile the sand into a large dome or other shape, depending on your project. The sand should stick together and hold its shape. If it slumps down, your sand is too wet and you should add more dry sand. If the sand cannot hold a shape, it's too dry. Continue piling the sand until you have the form you want.
Smooth down the outside of your sand dome by running a flat spade over its contours. Remove any air bubbles or cracks by gently pressing down with the spade.
Cover the sand with thick plastic wrap to prevent any sand particles from sticking to the concrete. Weigh the plastic down with stones or other objects. If you are casting concrete using an object, place that object on top of your plastic wrap.
Pour dry concrete mix into your bucket. Mix concrete with water in a bucket using a hoe or trowel. Follow the ratio on your concrete mix when adding water.
Scoop concrete from the bucket onto your sand cast using a trowel. Place the concrete over your sand cast in even layers, spreading out the concrete without tearing holes or creating air bubbles. Work until you have reached the desired thickness for your project.
Cover the sand cast with plastic wrap once you've finished working. Allow the concrete to dry for 48 hours. Check the sand cast after 48 hours; larger projects or humid weather will extend the drying time.
Remove the concrete from the sand cast when it feels dry to the touch, not moist. Pick up the concrete gently and remove it to a dry area to cure for one week. Curing will strengthen your concrete.