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This method is one of the most common ones for styling a bob, and it starts with your hair being wet. Spray or wet your hair if it is dry, or leave it wet after your shower before doing anything else to it. The blow dryer should always point down the length of your hair because pointing it at or up the length of your hair will result in frizz.
Dampen your hair or leave it wet after a shower. Place the blow dryer at your roots above where the round brush sits and aim it down toward your ends.