Sewing Facing and Interfacing
You will probably have three facing pieces for the neck edge: one front and two back facings. You may have two facings for each armhole as well. Gather these pieces, and remove the tissue patterns. Your guide will probably suggest interfacing for these pieces, which will keep the neck and armholes from losing their shape.
Cutting the Interfacing
Your guide probably tells you how to lay these pattern pieces out on the interfacing. Here's a tip that saves a step. Lay the fabric pieces out on your interfacing. The wrong side of the fabric should be against the interfacing. If you are using fusible interfacing, have the fusible side up, toward the fabric.
Pin the facing pieces down, and cut out the interfacing. Your facing pieces are ready for the next step. For fusible interfacing, trim about ½″ of the interfacing away from around the edges, then follow the manufacturer's directions to attach the interfacing.
For nonfusible interfacing, stitch ½″ from all the edges. Trim the interfacing ¼″ from the outside edge and close to the stitching the rest of the way around each piece.
If your fabric is very flimsy, and you are not sure you are laying the pieces out flat, use the pattern pieces to cut the interfacing, then pin the interfacing to the wrong side of the fabric pieces.
Sewing the Facing
Use the pattern pieces to help you identify the neck and armhole facings if necessary. Sew the neck facings together at the shoulders, and press these seams open. Turn the outside edge of the facing under ¼″ or along the trimmed edge of the interfacing, and stitch to finish this edge.
Sew the front and back of your dress together at the shoulders, and press the seams open. With right sides together, pin the facing to the neck edge, matching the shoulder seams and the center front. The back of the facing will extend beyond the zipper.
Sew the facing in place, and clip the curves. Grade the seam allowance by trimming one layer shorter than the other. This will help the seam lie flat.
Finishing the Neck Edge
Turn the facing to the wrong side of the dress by first pressing the facing over the seam allowance. This will crease the facing at the seam, making the turn neater. Press the neck edge with the facing on the inside.
Turn under the ends of the facing, and hand-stitch them down on either side of the zipper. Tack the facing to the shoulder seams. That is, make a few small stitches through the facing and through the seam allowance where they cross. If the facing is lying on top of any shaping seams or darts, tack it down there as well.
Depending on the style of your dress, you may want to topstitch ¼″ from the neck edge. Be sure you have pressed it smooth before you do this.
If your armholes are faced, they are done much the same way. Sew the side seams of the dress and sew the facing into a circle, turning the edge as described for the neck facing. Sew it on, clip, grade, turn, and press.
An alternative is to leave the underarm seam open and attach the facing first. Clip and grade the seam allowance, and press the interfacing away from the dress. Sew the side seam and the facing's underarm seam as one long seam. Press this seam open, and turn the facing to the inside. Tack it down to the shoulder and underarm seams.
Be sure to follow your pattern guide for order of operations since your pattern may have ties, pockets, or other things to add that are not described here.