I've never had any issues with it untying, I just knot it like I would a shoelace or ribbon, after wearing it for a few minutes just pull the knots tighter and it should be fine!
Be super careful. I bought one of her bracelets from Bergdorfs and lost it two days after purchase. Was a very expensive two days It was knotted and must have just unraveled and I lost it. As much as I love her bracelets I will not replace it. Still a little sad about the whole thing.