Make a diagonal stitch across four threads and down one thread from top left to bottom right.
Bring the needle up two threads below where you started and make a diagonal stitch across four threads and up one thread; take your needle down in the same hole as the end of previous stitch.
Work the next vertical row by completing the diamond shapes: stitch a mirror image zigzag which shares the holes of the first row.
Structure of stitch
Various Authors, The Royal School of Needlework Book of Embroidery (2018) , p.154