Understitching is a line of sewing that doesn't show on the front of the garment but holds facings flat.
1) Attach your facing in the normal way (below: a neck facing stitched to the neck of a tunic top) Press the seams towards the facing. It's easier to do this from the right sides. The neck facing and seams will then be together and away from the rest of the tunic etc.
Pin the facing to the seam behind it to keep it all from moving when you sew.
From the front, sew along the facing and through the seams. Keep as close to the edge as you can. I have used an 1/8 inch seam but it can be bigger if it's easier. I use that little red dot on the machine foot as a guide.
You will end up with this. Your new second row of stitching will be below your seamline.
Fold over and press. Your new sewing will be on the inside and won't show on the front.