Simmer thick leaves in a baking soda mixture. It should leave behind the skeleton of the leaf which you can then lightly paint or dye. Or you could always just buy some skeleton leaves in craft shop;
Hand-Stitched Felt Fox Coffee Sleeves - Lia Griffith DIY Cozy Felt Fox Coffee sleeves from MichaelsMakers Lia Grifftih
DIY Color-Tinted Skeleton Leaves- A free step-by-step tutorial, with photos, to make your very own skeleton leaves.