Lay the dark fabric face down on the background fabric and sew along the diagonal. Remember to sew just slightly off of the diagonal toward the corner so you can iron it over and have the corners meet. Trim away the bottom two layers leaving a quarter inch seam.
Now for the double flip. Place the small square of background fabric in the corner of your flip triangle, sew along the diagonal, then iron it over, and trim. You have just done a double flip.