Place one part dry Portland cement and one part fine sand into a large bucket and mix to combine. The exact amounts will depend on the size you want your finished project to be, but start with approximately 1 qt. of each.
Mix together one part cement fortifier and one part water in a second bucket. Add the fortifier mixture to the Portland cement mixture, stirring as you go. You want to create a thick paste. You might not need all of the fortifier mixture.
Place a drop of the concrete mixture onto the bottom of your chosen leaf, using your hands. Pat the cement onto the leaf, covering the entire leaf. You want a layer that is 1/4 inch in thickness. Add additional concrete mixture as needed. Place the leaf vein side up so that the mixture will pick up the shape of the veins. Repeat this process, applying concrete mixture to all the leaves you wish to set into your concrete birdbath.
Leave the concrete mixture to dry for two days, undisturbed. Remove the leaves from the concrete and discard them.
Pour a 5 lbs.of fine sand onto your work surface. Use your hands to mound the sand into a bowl shape. If the sand is dry and loose, add water and mix to create a pliable consistency, similar to the texture you would use to build a sandcastle.
Place your dried concrete leaves onto the sand mound with the vein side of the leaves facing the sand. Adjust the leaves until you have a pattern you like.
Prepare another batch of concrete mixture and concrete fortifier. Use your hands to transfer the concrete onto the sand mound, covering the mound and the leaves thoroughly. You are looking for a thickness in the center of approximately 3/4 inches. Make the cement mixture a little thinner on the edges, approximately 1/2 inch.
Leave the concrete covered mound to cure for the next week. After the week is up, carefully turn your concrete birdbath over. If necessary, use a garden hose to spray away any sand from the inside. You can also sand the sides of the bowl using coarse grit sandpaper.
Place your finished concrete birdbath on a store-bought base, or lay it directly on the ground for a more organic look.