This tutorial uses a knot known by many names; Josephine, Double Coin/Prosperity and Carrick Bend. It creates a decorative strip that can be easily finished into a bracelet or bookmark depending on your fancy.
Its takes a couple of goes before the knot makes sense and its one that while not complicated, takes patience to get right. Although you can use any cord to do this, I would recommend a minimum of 2mm the first time you try as it makes it simpler to learn.
PS, I’m using a piece of foamboard as a base, with pins to hold the top (and other parts of the knot if needed) in place, but you can use a foam macrame board, clipboard or just tape it down on your work surface.