Choose the large leaf for your birdbath. Large sycamore or oak leaves are good choices for a birdbath bowl.
Pour the sack of sand into a pile, and put plastic wrap over it.
Set the leaf face down on top of the plastic wrap. Cut off the stem of the leaf close to the base with a pair of scissors.
Mix the concrete and water in the wheelbarrow per the concrete directions. The mixture should be a thick paste--not too runny, but not so thick that you can’t mold it.
Put the concrete onto the center of the large leaf, and spread the mixture outward to the edges of the leaf. Carefully cover the large veins of the leaf well with the concrete, and be especially careful not to rip a hole in the leaf.
Make a lip or rim for the birdbath by cutting a long cardboard strip about 1/4 inch wide. Lay it around the outer edges of the leaf, and try to blend it into the large leaf with the concrete. Lay plastic wrap over the leaf birdbath, and give it at least 48 hours to harden.
Turn the birdbath leaf bowl over carefully, and take the plastic wrap off by gently pulling it away from the leaf birdbath.
Paint and seal the leaf birdbath if you want, and set it out by a pond or under a tree.