If the leather is really stiff, a brief soaking in cold water will help. Unfortunately it will probably shrink and turn hard when dry, so avoid doing this if possible. Make two small slits on either side of the slash at the instep, as marked on the pattern. Thread the thonging through and try on the shoe. The shoe is dyed and dressed.
Different styles are possible.
For right foot. Knotted thong in situ. Dated 10th century. All rights to materials herein belong to the original owners.