In sewing, one of the most common ways of finishing a raw edge is simply to turn the edge and sew it in place leaving a folded, clean edge. This technique works great on edges that are mostly straight, but it doesn't work well on a curved edge like a neckline because of the geometric principle that the inside edge of a curve is shorter than the outer edge of the curve (think of the lanes of a track). So, when you try to fold over a curved edge, there is not enough material available to turn and the fabric twists and pulls out of place.
There are three basic types of facings: shaped, extended and bias facings. Shaped facings are cut as a separate piece to match the shape and grain of the edge it is intended to finish. Extended facings are cut together as part of the garment and folded into place. Extended facings are frequently used to create the button placket on a shirt. Bias facings are created from bias strips that are sewn on and then folded over. Being cut on the bias, they can stretch to conform to curved edges.
Sewing in facings may be intimidating in theory, but in practice it is a fairly straightforward process. Most commercial patterns include instructions for sewing in facings as part of the pattern instruction set, abd once you understand how they work, they are easy to sew. It is important to do some planning ahead of time so that you will have the correct materials and tools on hand.
You will need to decide what type of fabric and interfacing to use for the facing and how to finish the raw edges as more fully explained below.
Facing Fabric:The garment fabric will dictate what type of fabric is used for the facing and interfacing. In almost all cases, it is appropriate to use the garment fabric for the facing fabric as well. In some cases, however, it is better to use a different fabric on the inside. For instance, if you are using a heavy fabric, you can use a lighter weight fabric for the facing. I have also used a different fabric for the facing on occasion when I didn't have enough of my garment fabric.
Interfacing:Most shaped facings and some extended facings require interfacing where bias facings do not. Facings provide a lot of structure to the garment and are subject to some stress so it is helpful to interface them with either a fusible or sew-in interfacing that matches the fabric weight. Cut the interfacing using the same pattern you used to cut out the facing fabric and apply to the facing fabric. If using a sew-in interfacing, baste it in place before assembling the facing pieces together.
Edge Finishing:The edge of the facing opposite the seam will need to be finished in some way to prevent the fabric from raveling. The best method to use, especially on curved edges, is a single-layer finishing method such as serging, pinking, or zigzag stitching the edge. You can also turn the edge and sew it down, but this can create bulk that might show through the garment. It is a good idea to do a test and see which method works best for the fabric that you are using.
Once you have decided what fabric, interfacing, and finishing method to use, you can cut out your facing and interfacing pieces. Stay-stitch the curved edges of the facing if necessary and apply the interfacing to the facing piece, baste or press in place. If you have multiple facing pieces, sew them together, press open the seams and then finish the raw edges.