While I have read about different techniques used, this class taught how to use a very lightweight fusible interfacing with the pattern drawn to that. This requires stitching from the back side of the fabric.
This is a small excerpt from Wikipedia if you are interested. I met a lady, years ago, who did traditional work. She explained that this stitch was used on Japanese fishermen work coats as a mending technique. Using many different configurations, the stitcher's applied patches and joined fractured seams to prolong the life of fragile, precious and worn fabric.
Sashiko embroidery was used to strengthen the homespun clothes of olden times. Worn out clothes were pieced together to make new garments by using simple running stitches. These clothes increased their strength with this durable embroidery.
Mostly geometric patterns are used to make this work. The common motifs used are waves, mountains, bamboo, bishamon, key fret, double cypress fence, arrow feathers, seven treasures, pampas grass, overlapping diamonds, linked diamonds, lightning, linked hexagons and persimmon flower. The embroidery uses special sashiko thread and needle. Modern day sashiko stitching is not restricted to the traditional indigo coloured fabric but uses a variety of colour combinations, and it is considered a beautiful surface embellishment for fabrics.
Many sashiko patterns were derived from Chinese designs, but just as many were developed by the Japanese themselves. The artist Katsushika Hokusai (1760–1849) published the book New Forms for Design in 1824, and these