Double Backstitch / Shadow Stitch
This stitch is worked as two lines of Backstitches, alternating between the rows. When stitched on fine or sheer fabric, the understitches show through (in the drawing, dotted lines), making a lovely shadow effect.
When you are working Shadow Embroidery, the shape that you want to stitch is rarely that of two parallel lines. More often, it is a curve - a leaf or a petal. When one side of the shape is longer than the other, you will need to adjust the size of your stitches so that the same number of stitches covers both edges. Notice that there are 8 stitches on each line, but the stitches on the top row are slightly longer than those on the bottom row. This lets the criss-crossing in the back cover the shape more evenly.
Back to Cross Stitches
The stitches should be small and even. The ensures that the shadow (the understitching) is fairly dense and even.
Make sure that two adjacent stitches on a row share a hole. If they don't, then the outline of the shadowed shape will look like a dotted line.
The shadow will be lighter that the thread, because it it showing through the fabric. You may need to stitch with a thread darker than you might guess in order to get the desired shadow color.
Don't try to work Shadow Embroidery on a shape that is too wide. The understitches will be too long, and it will be too hard to get a nice-looking shadow. Break the shape into smaller shapes. If you can't do that, consider using a Shadow Applique instead of Shadow Embroidery.
An alternate to the Double Backstitch, you can stitch a shadow shape from the wrong side of the fabric. When you do, the stitch used is the Closed Herringbone Stitch