usually to start with a straight edge to line up with you fold in half...fold salvage to salvage, line your ruler up along one edge using your fold as your straight edge, trim the first edge. now you have one cut straight edge. then turn, line up with the straight edge and trim the next side...continue around the quilt. if it were larger you would spread it out flat, line up with a straight edge in the quilt (a block or border usually works) and trim one edge at a time. or you can draw your straight line along each edge then follow your straight line to cut your straight edges.