I paper-pieced the center, and paper-pieced the 4 outside pieces. I made 2 of the outside pieces longer than the others, for long straight seams to the corner.
Looking at picture 1 above, you may note a problem. The sides of the center part are a little short! Guess what, I didn't notice this until I had it all sewn together, and found out it was too small. Ha ha indeed.
On the Dear Jane list, I've heard it repeated, "When in doubt, applique the #^%$#$ piece on." Which I was going to do - press the seam allowances of the 4 outside pieces under, applique....
But I got impatient. I lined up the center piece with that blue triangle, and sewed just that little straight bit! I left the needle down, sewing foot up, adjusted the top and bottom, held them together, foot down, and sewed to the end. Then, pulled the piece out, adjusted the two ends that hadn't been sewn together yet, pinned them, placed so the needle was at the beginning of the first straight bit, and sewed away.
And you know what? I fingered pressed the seam and it lay flat. And I was deliriously happy. I sewed the other 3 outside pieces on. THAT'S when I noticed one direction was too small.
Should I leave it, to remind me of my false pride? Nah, I'll take it apart so that that it looks good - and to see if I can do this Y-seam business again.