I have a stash that will outlive me 3x's over! I always start there with what I think will be enough yardage to do the entire quilt. On rare occasion I don't guestimate correctly. and have to make a single piece or two from something else and call it my one error in the quilt. I usually have a plan of sorts when I start a quilt. I know what block or blocks I want in the quilt. Other times they are samplers all with a primary color in each one. I usually have some direction before i begin.