For the triangles that go on the edges of the quilt
Measure your finished D9P block (9")
Add 1 1/4" to that measurement and cut squares (10 1/4")
Cut those in quarters diagonally
You will get four triangles from every 10 1/4" square, so you can figure out how many squares you will need.
If you want a little extra "fudge" space, just cut your squares a little bigger (perhaps 11").
For the four outer corners, add 7/8" to the finished size and cut 9 7/8" squares. Cut those in half diagonally. You will need two squares for that. Again, you can cut the squares a little bigger if you like.
The reason you need to do it this way is that you want the outside edges of your quilt to be on the straight of the grain. It avoids the wavy edges, so the quilt will lay flatter.