Start by parting the hair down the middle. I actually start my part on the side, then cut in to the middle about halfway back. I like side parts better, so kill me! Secure one half of the hair with a temporary elastic. Next, you are going to separate the first side into your five sections, but combine the top two and the bottom two sections just to hold them out of the way. You want to start in the middle to get the placement of your bun right.
French twist the middle section, rolling to the back, and stop where you want your messy bun. Then, separate out the second to bottom piece of hair.
French twist the second section of hair, rolling it up. Join the two twists together.
Continue with the bottom french twist, again rolling it up, and join it with the other two twists.
Next, you are going to do the top two french twists. French twist them both to the back, like you did with the middle one.
When you get the point where you stop adding hair to the french twist, continue twisting the hair until it will join with the rest of the sections. I didn't do this with the very bottom twist and it looks a little goofy, but...whatever!