I have just stitched this class sample for a class later in the year. Its funny having a day with no colour. The technique is one I learnt at the Royal School of Needlework when I was training. You have 3 layers of fabric which you stitch together then cut back to create tones of white. When I was training we had fine net in the centre, I use organza when I teach this as it is really easy to cut a hole in net. At least organza is solid, so you can't get your scissors caught in a hole.