You would only need to use the iron on interfacing on the backs of the squares that are stretchy. I would cut your squares then iron on the interfacing. I would cut the interfacing slightly smaller than the squares by say 1/8th of an inch since you don't want to end up with iron on interfacing on your ironing surface. The cotton fabrics won't need it. Then just piece it together the way you would any quilt. Hope this helps.
Debbie