I would handsew them together because of the bias.
1 On each piece I would mark the intersection of the ¼” seams at the center and the other corner with an “X” using a very, very thin pencil line.
2 Insert needle through the "X" on two pieces and make one stitch towards the outer edge.
3 Take a back stitch to lock seam and sew outwards.
4 Do not sew the last ¼”. This must be left open for the Y-seams.,
5 Put together in groups of three.
5 Create a half by sewing two groups together in the same fashion starting at the “X”.
6 Sew the next two groups together to make the other half.
7 Hold the two halves together by pinning in the outer area.
You need the seam area loose enough to sew from the “X” outward.
Sew one half to the end.
Go back to the center, start at the “X” and sew to the other end.
8 Press all seams to the side in a rotation.
In reality you will be better off putting the needle one thread away from the "X". You've got twelve pieces to put together and this will give you the ease you need.