I saw this on a YouTube video where these concrete leaves were being used to create amazing water features. Mine will be destined for a fairy garden I hope to create at the bottom of our garden! It’s on my To Do List ; )
I used native Puka leaves as they have quite prominent veins, which will show in the casting. I laid the leaves over heaped piles of sand, and sprayed them with oil.

Puka leave laid out over sand piles
I made up the concrete using sand as my aggregate, Portland cement and enough water to form a ball, the desired consistency. I patted the concrete to form the casting, over the leaves, using gloved hands.
I let them sit over night to harden.
I tried this method using a latex rope mould, and created a very unique bowl!