If you're making the Mathilde blouse, for example, staystitch the neckline on the yoke and back bodice pieces.
Do your staystitching on each piece of fabric after cutting them out but before attaching the pieces to each other. It's a good idea to do it as a first step, as simply handling the fabric pieces can cause them to stretch out. Stitch a line parallel to the cutting line, within the seam allowance so you won’t see it from the outside of the garment - on a pattern with a 15mm (5/8in) seam allowance, you should staystitch about 10mm (3/8in) from the raw edge, but check the instructions for the pattern you're making in case a specific seam allowance is cited.
It’s a good idea to sew in the same direction on either side of the centre to keep things symmetrical. So on a neckline, for example, sew from one shoulder to the middle, then from the other shoulder to the middle.
And that's it!
Read more about sewing the Mathilde blouse...