Staystitching is a row of machine stitching applied anywhere along the seam allowance to stabilize and strengthen sensitive areas and special seams both in the construction process as well as within the finished garment. You should not underestimate the power of a simple stay stitch, especially when working with fabrics that are prone to stretching and loosing their shape, especially in the sewing process.
Regardless of fabric type, it is standard dressmaking practice to staystitch curved fabric edges as soon as you unpin the cut fabric pieces form their corresponding patterns. This is due to the fact that curved fabric edges tend to stretch out of shape as the garment is being constructed. As a result, if these sensitive edges are not staystitched for stability, the neckline or armholes will not lay completely flat in the finished garment resulting in a less professional finish. In addition, you may find it difficult to work with an edge that has stretch out of proportion as it will be difficult to align with other corresponding edges.
If you are a sewing beginner, you may be asking yourself when and which edges should you staystitch. As a general rule always staystitch the neckline edge, curved waistline edges, and the curved most part of the armhole. As you garner more practice and experiment with different types of fabric, you will be able to make your won judgement as to when a staystitch is necessary along which edge. It is common that when working with loosely-woven fabrics that fray easily when cut, most edges are staystitched to preserve the garment pieces's shape and stability as it they go through the sewing process.
Staystitching is applied within the seam allowance and should not visible in the face of the finished garment. As a rule of thumb, apply the machine staystitch about 1/8-1/4" away from the seamline. If your seamlines have been transferred from your patterns, use it as a guide in the application process. If you do not have seamlines marked directly on your fabric pieces, use your machine's seam allowance guides to apply the staystitching evenly at a short distance away from the designated seamline, about 1/8" less than the seam's designated seam allowance.