Also known as handpleating the old way. At a time when you decided about 2 years ahead or so to get the wool dyed and pleated for your new outfit...   
A renaissance dress     Farging   Folkcostumes      Pleated linen     
Different versions of pleating is known to have been used as far back as the viking era. Handpleating continued in use untill about the beginning of the 21st C when the method was transplanted by affordable machinepleated fabrics.
To make a pleated skirt you would need 3 or 4 lenghts of white undyed  and unbleached wool fabric.
Pleat as shown. Wash the wool. Store it for about a year.
Rub the pleated wool with liquid green or brown soap, put it in a boiling iron cauldron. Repeat process. Rinse and boil.
The wool is colored black using ironvitrol (jernvitrol), coppervitrol (koparvitrol)and something called brissel (no clue). Rinse, hang up to dry. Store for a year with the threads in the wool.
Remove the thread. You now have permanently pleated wool with little holes showing where the thread was.
Make your outfit.
These pictures are from a demonstration 25 years after the last woman known to do this kind of pleating died (1942). Her daughter demonstrates the way she remebered her mother doing it. 
Sewing the pleating threads
Tightening the threads
Preparing for storage
Pleating pattern with finished examples
Advaced pleating pattern
Closeup of undyed pleated section
Examples of undyed and dyed pleated wool fabric in Norsk Folkemuseum, Oslo, Norway
A different pleating methode is used in Hardanger. In the Hardanger villages they have used "felte","fodla" and "rukkastakkar". Felte (pleated)being the oldest. For this "venda-vadmel" (kypert) would be used as it retained the pleats the best.
The pleating was done with a flat board and a flat stone. The fabric would be put on the board and 2 long , thin pieces of round wood would be inserted. They would be put under and over the fabric, interchangeably like when pleating headlinen.
The fabric would be moistend piece by piece ( with boiling water according to some) and squized thightly thogether. Threads would be sewn through the pleats as the fabric was being pleated, bit by bit (the color was either added then ,or there is a typo). The fabric would then be put under pressure using rocks and left to dry in the sun.
When the skirt fabric was dry, it would be permanently pleated. The thread would be removed and the skirt sewn up using backstitch. The skirt usually consisted of 4 lenghts of wool sewn into one large piece before the pleating process. The front piece was not pleated and was left flat under the apron to avoid a bulging stomach.