The ‘snake tongue’ is a medieval fastening system and consists on a leather strap that is subdivided into two parts, like the tongue of snakes 😉 to allow you to knot it to any surface.
Like any fastening system, they usually have two parts, one (let's call it male), the aforementioned bifid part and the other (let's call it female) which is where we are going to knot the snake's tongue.
Traditionally it was used in belts, when it was not so easy to get a buckle. We usually use it a lot for articulated areas as fastening of shoulder pads to harnesses or ruffles because it allows a full mobility and the ductility of being able to knot the ends of the tongue to any surface without any added problem.
To make a good knot you can either follow the instructions in the image of this publication or let yourself be intuitive and look for your own style. As a tip, never sacrifice mobility if you want to give it a personal touch.