As the name tells, this is a stitch exclusively used to make leaves. The simple satin stitch is used in a particular way to fill in a leaf to give it a more realistic appearance. Therefore, this stitch is used commonly in crewel embroidery and, especially, in the Brazilian embroidery.

If you know the satin stitch, this tutorial will be super easy! I will be working on a leaf pattern divided into two sections vertically.

leaf stitch 1    leaf stitch 2
Fig 1: Trace a pattern of leaf and draw the middle vein as shown. I have marked the stitch lines as A,B,C.
Now start working on the satin stitch between the stitch lines A and B. 
 Fig 2: Start from the broader end to the tapering end. Make sure not to leave any spaces between two stitches, and to keep the edges as neat as possible. 
leaf stitch 3  leaf stitch 4
 Fig 3: Once you finish one side, turn around and begin the next side. You will gradually see that the vein of the leaf shows up, giving it a realistic appearance.   Fig 4: A finished leaf would look like this. You can try to make the leaf with different shades on either side to get a different effect.