I took a class on Stack and Whack and you have to do the same pinning thru layers. She suggested using a half inch to three quarter inch piece of styrofoam sheet. Worked like a charm no
slipping or sliding and just stick the pin in the same place all the way thru the layer into the styrofoam and when you are done,
use big straight pins to secure the fabric layers together as you
leave the pins in the foam until all is very secure. Sorry I am not a very good explainer. Good luck it is a lot of fun.
Karen /wichypoo