Sushi,
If you look at my pic above you will see the pink square beside the purple one. The square where you would put the purple one! Before I put the purple on one, I sewed the "triangles" that I joined at the crease from tip to tip. You see two "lines" on the square - one of them is the seam in the middle where they were joined, the other is the the seam I sewed to keep them in place ...
does this help? If not, let me know and I'll take pics tonight!