I generally use inexpensive muslin for foundations. I do prewash it before cutting, cut my squares ( or rectangles) a couple inches larger than I want my blocks - that gives me plenty of room to play with my scraps- then when I get all my blocks finished I trim them to all the same size.
Sometimes I find I have many scraps of white, or a light fabric that are long strips maybe cut from quilt backings and I use those for foundations then it’s all scraps in the quilt.
