Very pretty! Have you heard about the gelatine method? You make up a weak solution of gelatine and water and dip your misbehaving fabric in it, let it dry and then it's slightly less wriggly when you cut it out and sew. I tried it with some slippery fine silk crepe and it worked very well. You just rinse it out when done.
My link won't work but if you Google it, there are a couple of recipes online. HTH.