Before mixing the concrete, make sure the mold is clean and clear of dirt. If it has been used before, clean it up with soapy water or denatured alcohol. Using mold release is recommended but not necessary.
Simply brush a little bit of light vegetable oil or mold release in the form.
If the oil makes little puddles, you've used way too much. Soak up the excess with a paper towel.
Too much mold release will make air bubbles in the surface.
Not using mold release can make the stones more difficult to de-mold, and you'll need to be more careful about damaging the mold.
Now place the mold on a plywood base board for the remaining steps. The board can be anything rigid, but should at least be flat and wider than the protrusion of the mold.