When you're sewing with a stretchy fabric such as Jersey or any other loosely woven fabric, stay stitching stabilizes the edge of the fabric and prevents it from pulling out of shape during sewing. Stay stitching is done on a single thickness of fabric with a line of stitches running inside the seam allowance, so it is invisible when the garment is complete. Stay stitching is also a useful technique to learn if you want to create a controlled frayed hem; the stitching will prevent the fabric from fraying beyond the desired depth.
Cut out your pattern as normal, following the instructions for the layout of the pattern pieces on the fabric. Mark up the pattern for any sewing instructions, such as positioning of darts.
Stay stitch each piece of fabric individually. Stitch 1/2 inch from the edge of the fabric. Stay stitch the shoulders of blouses, starting at the neckline and stitching towards the armhole. Stitch around the armhole from the shoulder down to the armpit, and stitch the sides from the armpit down to the waist.
Stay stitch necklines in two sections. Work from the shoulder down to the center of the neckline and stop sewing. Remove the fabric piece from the machine and start sewing again at the opposite side of the neckline, working again from the shoulders down to the central point.