If I am making a "pattern quilt" that I have seen, I cut out all my pieces first. If it is a scrappy quilt, with leftovers, here is what I was taught to do. Cut up all your extra pieces in various sizes, for instance, squares that are 2" or 3" or 4", whatever you decide. Or you might cut up strips in various widths and lengths. I put them into shoe boxes that are marked with the correct size. When I go to put all these pieces into a quilt, I throw them all into a large basket (keeping in mind that they should be the same size) and just grab whatever my hand hits. Then I sew. I used to be very structured and had to have everything lined up. But, a scrappy quilt needs to be just that - Scrappy! Don't worry about the colors matching. Somehow they always look good when I am finished.