STEP 1: Tease your hair(teasing hair is the key to master the messy bun). if you have straight hair you need to tease them more than if you have curly or wavy hair. Tease your hair by back combing them majorly at the crown area of your head to give that messy look.
STEP 2: Flip your hair upside down to give more volume to it. If your hair has enough volume you can skip this step.
STEP 3: Now roll your hair and when you reach the end make it into a bun. It is a messy bun so it should be loose not that tight.
STEP 4: Fix using hair/bobby pins. Take out few strands of hair from the sides to rock your messy bun!!