I'm sure I'm not the first one who found it out. Well ... one day I will do momentous discovery.
What do you need?
1. Your wavey crochet square
Simply pin your crochet squares to the pillow. Give the shape/size which you need. Hold in pins for one hour (more and less). I did not use iron or water.
On right side, in deep blue coulour you can see ready to use/sew square.
Have fun !