After completing the exercises from Envisioning Your Customer, you should have a decent idea of who you think your target market is. Review your findings, and create a list of places you think they can be found, so that you can conduct your interviews. Use your persona to brainstorm where people like this can be found. You want to find a place where you and your team can go to intercept people to talk to.
These places can be a physical location like a mall or a coffee shop, or online using social media, chat rooms, or other online resources.
Visit the places on your list and look for your customers. Observe them and their surroundings, and formulate questions that will give you insights into their behavior.