Start by getting your leaf pattern ready. You can draw or print on a piece of paper. Lay a piece of freezer (shiny side up) or wax paper over it and tape down so it doesn't shift.
Roll up a golf ball sized amount of yellow and white mold putty. Combine them thoroughly by pinching, "smooshing" and rolling until it's a uniform yellow color with no white parts.
Roll it into a snake that's a little thicker than a pencil. Bend it around on the paper to follow the leaf shape. Pinch ends together where they meet, trying to smooth a bit. Press it all down a bit too, to make sure it has flat contact with the freezer paper - but don't lose height.
Let the mold putty set for at least one hour. Then put the leaf pattern under the silicone mat and move the putty rope over to the silicone. To keep resin from leaking out under the mold, add a line of hot glue all around the outside of the mold.