Leaves must be completely dry. Melt beeswax in a double-boiler. Can use old candle stubs. (Optional - add a few drops of cinnamon oil). Hold leaves by the stem and dip in wax. Let excess drip off, then lay them on waxed paper to dry.
This is how you get really smooth, lovely satin stitches. Outline in split or chain stitch, fill in the area with chain or long & short stitches, and then do the satin stitch. The underlying stitches support the satin stitch and help it to lay smoothly.