I've done it both ways. Once in a while I'll find a pattern I love and then buy fabrics for it unless I have some in my stash. But usually, I buy fabric and then I will "bundle" fabrics that I like together for a future quilt. Then I look for a pattern for it. I usually buy 1 yard pieces except for fabric I intend to use for a border, then I buy 2 or 3 yards. Sometimes, I find that I only have enough fabric to make a throw or wall hanging, but I always thought I'd rather have a big stash of smaller pieces than a smaller stash of big pieces! Lately, I've been making a lot of scrap quilts, in a controlled color-way, so I will pull a pile of blues and whites, or burgundies and tans, or navies and creams. All those half yard and 1 yard pieces really go a long way when cutting a few strips off for a scrappy quilt.