It is very easy to do and is a play on a knot you can do with a normal square scarf.
I am wearing/using my Hermes Carre en Carre Pointu which I bought in Hermes Berlin Friedrichstrasse last January.
Then pinch together the right angle/square corner you have made along the longest part of the triangle
Then gently make a knot and pull tight - try to avoid straining or pulling on the hand rolled hems
Next flip the scarf ends open to reveal the right side again and tuck the knot down behind the twist of silk