Find a nice big leaf for your mould. It should not be flimsy since it will have to hold up to casting. Make sure it has nice veins. Good leaves are those from Elephant Ears, Pumpkin, or Hosta plants. Cut the stem off so you just have the body of the leaf.
Empty your sand into a large box. If you prefer, you can put it on a piece of plastic on a table. Make a mound out of the sand and dampen it with water. Cover the sand with a piece of plastic.
Mix your concrete according to the directions. It should be thick like cake batter.
Place your leaf on top of the plastic covered sand, vein side up.
Cover the leaf with the concrete mixture. Make it about and inch thick all of the way around the leaf, starting in the center and working out.
Cover the concrete on the leaf with plastic and allow it to sit unbothered for about 2 days.
Remove the plastic after it has had time to cure. You can now flip the leaf over and peel off the original leaf.