Pick a pattern and choose your colors. I'll do a simple chevron. You're going to want to make the color of your first string longer than the others. In this tutorial, purple is the first string.
Fold your strings in half and find the middle. At the middle, put your strings under a clipboard, or pin them, and make sure your first string is first.
Repeat doing forward and backward knots until you get the length you desire. Make sure your string ends back on the left side when you're done.
Once you're done with that side, turn your strings upside down so you can work on the other side. The starting position of your first string should be on the right. Repeat doing forward and backward knots with this string until you get the length you want. Instead of starting with a forward knot like on the other side, you will start with a backward knot. Make sure your string ends back on the right side when you're done.
When finished, fold your loop so the ends meet and make sure your strings are in the correct position. If they aren't, just move them until they are.
I made each side of the buckle 1/2in long. My longer string was 18in, folded in half that's 9in (I didn't make it long because this is just a tutorial) and at the end, it was 6in on both sides. Doing 1/2in worth of knots takes up 3in. Take this into account when measuring and cutting your strings.