The best way to do a messy bun is to put into a loose ponytail right where you want the bun to sit. Use a scrunchy, or other large holder that has good elastic but won't tie your hair too tight. Then begin wrapping your hair in a circle around the ponytail, taking care to not make it too tight! Tuck the end pieces inside of the bun and secure with bobby pins. After a few minutes your hair will start to loosen and fall out a little, giving it that "messy" look.
just simply don't try. lol have the ponytail holder on your wrist and pull your hair up where you want it, when you start to put the ponytail holder on, don't let your hair go into a full ponytail, then twist and wrap it again.
not sure if that makes much sense, it's harder putting it into words.. lol good luck! it's quite simple once you figure it out!