This works best if you use a pattern that has a mirrored string order.
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 and your last middle string before it mirrors, longer than the others. If you don't know which is your last middle string, look at the first half of your strings. Your last middle string is the last string in that group. In this tutorial, purple is the first string and green is the last 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 and your last string is last. Keep all the other strings in the middle (doesn't matter what order they're in.)
Pull up to tighten (it may be easier for you to tape down the middle strings so they don't move.) The strings should've switched places.
You will repeat steps 3-5, always starting with your original first string no matter which side it's on. After you do the steps a second time, it should be back on the left.
Repeat steps 3-5 to desired length. When you end, make sure your first string is back on the left where it started.
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 steps 3-5, using your original first string, until it's the same length you did on the other side. When you end, make sure your first string is back on the right.
When finished, fold your loop so the ends meet and make sure your strings are in the correct position. If the middle strings aren't, just move them until they are.
I made each side of the buckle 2cm long. My longer strings were 46cm, folded in half that's 23m (I didn't make them long because this is just a tutorial) and at the end, they were 16cm on both sides. Doing 2cm worth of knots takes up 7cm. Take this into account when measuring and cutting your strings.