Sorry to take so long in getting back to you. I made the dahlia part on a much larger backing fabric (in my case, about 22" square), so that I would have room to add a border. I cut eight pieces of border fabric - about 1.1/2 inches wide and long enough to cover the end of each of the 8 parts of the dahlia with a couple of inches extra on each side. I sewed opposite sides each time. Top and bottom, side and side, corner and corner and then the last corner and corner. I pressed the top, bottom and side strips open as I added them, then sewed the remaining four strips so that they overlapped the ones already done. When you press these open, you will find that the border is nicely complete all around. I then added the background fabric in the same way but using pieces 5" wide. If you want a larger pillow, you can make these pieces even wider.
I hope this is clear. It sounds complicated, but is really quite simple. Good luck.