How to understitch
Understitching is a great way to keep your linings and facings sitting nicely on the inside of your garment without the stitching showing on the outside.
All it requires is a line of stitching close to the edge of a facing (or lining) to keep it from rolling toward the outside. It comes in especially handy when sewing around a neckline. The example below is on the Fantail Shirt neck facing.
1. Attach the facing to the bodice neck per the pattern instructions. Clip the curve to ensure the curve sits nicely.
2. Turn the garment to the wrong side up and press the seam towards the facing. Pin in place.
3. With the right side up - stitch the seam surplus to the facing in a line close to the stitch line that you created in step 1 (between 4-5mm or 1/8″ to 1/4″ away from the seam line). Do not sew any part of the main bodice fabric - you are only sewing the facing seam surplus back against the facing itself. I used one side edge of the foot as a seam guide when sewing.
4.Press the facing towards the inside of the garment.
5. The inside should look something like the photo above. The finished neckline, right side up, will now lay flat with a crisp edge, preventing it from rolling out of place (see photo below).
Depending on the garment design and your fabric, you may still need to "stitch in the ditch" at the shoulder seams to ensure the facing stays in place whilst worn.