One important rule of cutting out a garment - follow the grainlines marked on your pattern pieces.  Typically, a grainline is intended to be aligned with the lengthwise grain of the fabric (parallel to the selvage).  This gives the stretch of the crosswise grain around the body.  A grainline may be labeled as "crossgrain" - perpendicular to the selvage, or even "bias" - 45 degrees to the selvage to give the most stretch possible.