Take care that the pieces are arranged according to how you want them to be placed in the block. In this case: dark will be lower left, light on top, etc.
Sew the medium onto the light, with a pin to mark where the seam between light and dark is.
Pivot at this point and then sew medium to dark, being careful that the dark remains in the proper allignment when you pivot, so that the seam turns exactly where light and dark meet and the medium and dark pieces match up exactly.
Also, be careful not to stretch the bias cut edges while sewing!
This strip piecing tutorial for Tumbling Blocks looks promising and i started to try it, but then realized it's better suited to a project that isn't so scrappy, i.e. with just three fabrics, like for a baby quilt. Also, i preferred the aesthetics of this method. After all, sometimes the viewer is not galloping by on a horse!