Sushi reverse applique is where your appliqued piece is under your background instead of on top, you fold the edges of the background back and sew it to the applique piece.
I don't know if this will make sense but I sewed the square to the trapezoid but stopped 1/4 inch from each edge. Then I sewed those sets to the octagon, doing the same stopping 1/4 inch from each edge. That allowed me to fold each edge that touches the diamond to be folded back on what would have been the sew line. Then I placed an oversized piece behind the diamond shaped hole and top stitched it in place.