Thread your machine with a toning multi-coloured thread. I suggest a Polyneon as it is easy to use.
Drop the feed dogs* of your sewing machine
Fit an ’embroidery’ or ‘darning’ foot
Set the stitch width for the widest zigzag
Set the stitch length to zero.
Stitch at random, moving the hoop smoothly and not too quickly.
Secure the edges and the threads .