An All-in-One facing is a facing that includes not only the neckline, but also the armholes. You cannot place the facing to the dress, sew along all the edges, and then turn it right sides out. It simply will not turn out. You will need to start by sewing the neckline, and then the armholes, step by step as I will demonstrate below.
It depends on your design or garment, but for this example, the front facing is cut on the fold, and the back facing is cut x2. This is due to the zip being in the back. A facing pattern is copied off the main pattern, so it will match the fit and style.
One you have cut out your pattern from the fabric, cut out the same in the fuseable interlining. Fuseable Interlining was described here. Once you have ironed it onto the wrong side of the facing, Sew the side seams and shoulders of the facing together. Iron the seams open, and the either zigzag or overlock the outer edges.
Place the Facing to the prepared dress. Match the neckline curve, pin and sew together at your seam allowance. You will see I have two sew lines- I sew curves from the left hand side, and to make sure I get the same curve, I flip over my sewing, and sew it again, from the right hand side.
Then, Clip your curves, but cutting simple snips into your seam allowance, usually 2,5cm apart. These snips allow the seam allowance to open up, which will allow you to turn your facing in nicely.
Iron the seam allowance to the facing side. Then you will sew a staystitch, which is a straight stitch sewn on the facing, holding the seam allowance. Sew this staystitch about 3mm away from the seam line.
Turn in your facing to the inside of your dress, and iron again.
Now you will be ready to sew the facing of the armholes!
Don’t be tempted to sew the facings as they lie now- all nice and flat. You will need to twist the facing to the dress, to get it right. The biggest tip here, is to sew it step by step: first the front part of the armhole, turn it out, then the back of the armhole.
Have the facing lying flat to the dress, wrong sides together. Then turn the facing to place it right sides together with the dress, at the underarm/ side seams point. Pin.
Now the facing will begin to twist. Pin the facing to the dress at the midpoint of the armhole, or at the notch. Do only the front armhole.
Then meet the shoulder points and pin. Now sew the facing to the dress, from the underarm point to the shoulder. No further! Clip curves, and turn out, back to having the facing lie flat with the dress (back to the beginning!)
Now do the back armhole in the same way. You will find that once you are pinning the back armhole, you will need to squeeze in all fabric, to make the facing to meet the dress armhole.
It is difficult to iron this curve, so go ahead and do your staystitching on the facing, holding the seam allowances. Start at the furtherest point that you can get your machine into, close to the shoulder point. Sew your staystitch all the way round to the back armhole facing, as high as you can sew.
Turn your facing to the inside of the dress, and iron.
You should have a beautifully finished armhole and neckline. The key tricks are to use fuseable interlining, clip your curves, and to staystitch the facing.