This stitch is worked from top to bottom.  To begin, work a horizontal stitch from left to right.  Bring the needle out just below where it went down.  Hold the thread to the right, making a loop to the left.  Slide the needle underneath the horizontal stitch and the top of the loop and over the bottom of the loop, as shown.  Pull the thread to the right to make a knot.

Repeat this process on the left hand side, leaving your thread slightly slack.  Take the needle down through the fabric just below this knot.

Leaving a gap, bring the needle up on the left hand side directly below the knot.  Taking the needle from top to bottom underneath the last horizontal stitch, catch the working thread as before to make a knot in the middle of this stitch.

Take the thread down on the right, directly opposite the start of the stitch on the left.  Bring the needle back up just below this.

Repeat these stages as required.