If possible, it would be best to work in a covered area with a solid floor: a garage, covered patio, or outbuilding. If not, you will need something solid, such as a sheet of plywood, for a surface to work on and a tarp to cover it when you are done. We set up to work on the concrete center aisleway of the barn. The weather was a bit iffy, and we could leave everything to sit and cure right in place.
We started laying down some forms to add depth and support to the sand piles. You won't need as much sand this way. We used styrofoam, plastic bottles, and stacks of wood. We did this because of the size of our project and because we really wanted some depth to the pieces, so they could hold water. This step wouldn't be needed for a smaller or shallower finished item. We then added the sand and worked it into a pile of the size, shape, and depth we needed. Some of our leaves were 4 feet long. Your sand pile may not be as large or have as much depth as ours. It will depend on the size of your project.
When the piles were the size and shape we wanted, we covered them with a layer of plastic to keep the sand out of the cement we were working. Then we put down the leaf; face down, so that the veins of the leaf would come into contact with the concrete. Our project was a fountain and I knew that I wanted a hole in one of the leaves, so I let the thick stem on one of them stick out several inches. The stems of the other leaves were trimmed off very short.
We started to pour some of the concrete over the leaves, forming it to the shape of the leaves, following the edges as we went. Because our leaves were so very large and our finished project would be so very heavy, we added a layer of extruded metal plaster lath, pieces cut to fit the shape of the leaves for extra strength, and then another thinner layer of cement over that. If your projects are smaller, you can skip this step.
We covered them with plastic and let them cure for 2-3 days. Curing depends on the size of the project and the weather. Then we turned them over and peeled off the remains of the leaves. This would be the time for any necessary touching up. You can use a metal file or grinder to clean up the edges. If you want holes in your work and forgot to put them in when you poured the concrete, drill them now. The longer it sits the harder it will get.
We made 6 castings, 5 of them were individual Portadora Alocasia leaves and we made a bowl shape with a castor bean leaf imprint in the middle.
There are several different ways that you could finish them. You could just leave the raw concrete unfinished, you could coat it with a concrete sealer, you could mix a color additive into the wet concrete mix before forming the projects, and you could paint it.
I decided to paint mine, using exterior, water-based enamel, porch paint. I bought 3 different shades of green. I coated the whole leaf with the darkest green, then dry brushed on the medium green, and finished up with the lightest green for accents. After they sat for a few days, the leaves were sprayed with an acrylic enamel sealer. These leaves were put out into the yard in the spring of 2014 and have been weathering there since. They have held up well with no signs of deterioration for 2 summers and 1 winter.
So get out there, start looking over those leaves, pick a few likely prospects, and start your project.
Plastic bottles, styrofoam, scrap lumber
Plastic trash bags
Extruded metal plaster lath
Cement mix = 1 part Portland Cement to 2 parts sand.