If you're making the Mathilde blouse, stay stitch 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. Stitch a line parallel to the cutting line, within the seam allowance - about 1cm (3/8in) from the edge if you’re using 15mm (5/8in) seam allowance - so you won’t see it from the outside of the garment.
It’s a good idea to sew in the same direction on either side of the centre to keep things symmetrical. Soon a full neckline, for example, sews from one shoulder to the middle, then from the other shoulder to the middle.
And that's it!