Shadow work as the name implies, is the shaded part of a design worked on the wrong side of a fabric.
Embroidery Thread (Double Shade or dark&light shade
First trace out the required design (usually flowers) on the wrong side of the cloth using carbon and tracing sheet. Take two strands of embroidery thread, put a knot at one end and start stitching.
There are two methods of stitching :
Bring the needle to the right side at 1, in at 2, out at 3, in at 4, out at 5 and so on.