The leaf stitch or fishbone stitch is ideal for making small leaves. It requires us to divide the pattern into two and each side is filled alternately giving it a plaited effect in the center. I suggest you to use just one thread to obtain the best rendering with this stitch.
Start at the top of the leaf with a straight stitch. Then alternately work straight stitches diagonally from the outside to the spine on each side of the leaf. To keep the angle of the stitches correct, when you enter the fabric on the center line, enter very close to the previous stitch. You should obtain a smooth surface, almost like a satin stitch.