whipped stem stitch 1     whipped stem stitch 2
Fig 1: Lay a foundation of stem stitch. Bring out another thread through the start point and pass it through the first stem stitch.   Fig 2 : Continue this winding action between each stem stitch.
whipped stem stitch 3
Fig 3: you should finally get a pattern as shown.

Make sure the fabric beneath is not plucked while whipping the thread around the stem stitch. Use a blunt needle to acheive this.