Pile up the sand in a convex shape. Wear gloves to protect your hands. Make the sand dome large enough to hold the leaf with about 3 inches of sand around the perimeter. The height of the sand will be only a few inches.
Cover the sand dome with plastic wrap. Anchor the plastic wrap in place with a small bit of sand around the edges.
Pick the leaf you want to use for the mold. Wait until this step to pick it so that it does not wilt. Alternatively, you can pick it earlier and keep it in water so it stays fresh. Cut the stem so that is it only 2 inches long.
Place the leaf face down on the plastic wrap. Make sure there is a cushion of sand covered in plastic under all parts of the leaf. Press it gently into the sand so that it is supported well and forms an even dome.
Mix the concrete in the bucket. Put the dry mix in the bucket and slowly add water as you stir. Mix thoroughly so you do not have any dry pockets of mix. The consistency of the finished mix should be like ice cream. You should be able to spread it easily without it dripping off the leaf.
Put about two or three cups of the concrete mix in the center of the leaf and begin to spread it toward the edges. Add more concrete mix as needed until the entire leaf is covered. The concrete should be three-fourths of an inch thick in the middle and one-fourth inch thick at the edges.
Cover large leaf veins well and do not leave any empty patches in the concrete. Keep in mind that this will be the sculpture, so whatever it looks like wet is what it will look like dry. Make sure there is no extra concrete hanging past the edge of the leaf.
Cover the wet concrete with plastic wrap and allow it to dry. This will take about 48 hours, depending on the weather and other factors. If the concrete is in full sun or the weather is hot, cover the whole thing with a damp piece of burlap to help it dry evenly.
Uncover the concrete and turn it over carefully. It has not cured yet, so be very careful because it breaks easily. Remove the plastic wrap from the top. The leaf will be stuck in the concrete. Remove that. If any parts of it sticks, use a nylon scrub brush to remove it. If large veins do not come out, wait a few days until they dry and they will come out more easily.
Let the concrete cure for one week. Do not let it get wet or let water stand in it during this time.
Seal the concrete leaf by brushing on concrete sealer. You can find sealer at home improvement centers. Now your weather-proof molded elephant ear leaf is ready to be put into the garden.