I start with the main fabric which will direct the colors. Then I pull fabrics to go with it trying to mix and match scale and value. A lot depends on the pattern I plan to make. I often fold the fabric so it will be close to the size of the actual piece just to see if the fabric comes through OK. When all the fabrics are selected, I lay them out on top of each other and step back. Often, I rearrange the order to get a better feel for the match. Or I squint to see if anything jumps out at me.