The colours are fairly randomly threaded, I’d guess a template was first worked out to get the holes exactly spaced, then a large darning needle used to push through on a soft background, like for stitching leather, the ideal thing being a cork sanding block from a hardware store, or Wilkos, which I’ve seen leather workers using, then just thread each colour one at a time fairly randomly through the holes from top to bottom, or whatever, cut and tape, or use a dab of fast drying glue, then do the next colour. Then stick a piece of matching card cut to size on the back, or use precut card and stick to already folded cards, as others have suggested.
You could do the black card one size, then stick onto folded cards that are slightly biggerer, in a contrasting colour, maybe matching one of the threads, giving an equal border all round.
Just a thought. 🙂