|This is a great tip I picked up from sewing La Fred's Athena Blouse. It works with either fusible or sew-in interfacings.
First, seam together any facing/interfacing pieces that will be sewn to each other. If you are sewing a top with a front opening and a jewel neck, for example, sew together the front and back facings and (separately) the front and back interfacings. Trim and press the facing, and finger press the interfacing.
Place facing and interfacing right sides together. Stitch 1/4 inch along the edge that usually would have to be clean finished, and a scant 5/8 inch at any place where the facing would be turned under 5/8 inch. For example, on a front opening facing that eventually is tacked to the shoulder seam, you should stitch 1/4 inch along the edge that will be free on the inside when finished, then pivot and stitch 5/8 inch along the edge that will be tacked to the shoulder. Do not stitch the bottom edge--leave it free just as you would normally. Trim and clip. Turn wrong sides together. If the interfacing is fusible, fuse it in place. If not, press and baste along the open side that will be sewn to the front edge. Then treat the facing as usual. This produces a beautifully finished facing, no serging, turning, or zigzagging needed.