So here you have your pony tail you want to make into a messy bun.
Pull it up 1/2 thru & secure with elastic
This is the secret right here: do the exact same thing as that first step. Fold the hair in half again & secure with elastic.
Then depending on the look I'm going for, I will add bobby pins to give it a more messy look - or use rubber bands & do the messy look I've always done. It's that second step that is helpful for those with longer hair.