It's easy. Just tell about a moment in your life you would categorize as your nerdiest. I'll give you two of mine.
I've yelled "Hoohaa, nerdfighters!" in class while they were reasonably quiet.
Fought with homemade lightsabers in the halls of our middle school with Star war attire on while yelling out quotes from the movies.
My bff and I once stole a bunch of cardboard boxes from Subway and basically turned my car into the Millennium Falcon.
Also, we took a bunch of old Star Wars figurines and made our own version of "A New Hope." Princess Leia had a southern accent, Han spoke in rhymes, and Luke arrived in his speeder thing to 70's porno music. Good times ^__^
It was silent reading in my class and I was reading New Moon. I was at a part when Bella was with the wolves and she was telling them something, I got really angry at her and practically yelled "No, Bella don't!" Everyone stopped reading, looked at me and laughed. Despite the embarassment, I still talk to my books... alot.
I also constantly say things related to Youtube or Nerdfighters, my friends always get confused and I've stopped explaining things because it gets tiring.