For the complete The Last of Us: Left Behind experience, read Prima's free walkthrough.
When you regain control of Ellie, follow Riley. Once you crawl through the door, be sure to collect the Wanted Poster off the desk at the back of the room. If you start climbing, you've gone too far. After climbing the rubble and shimmying across the broken floor, the first Optional Conversation is just ahead, near a Firefly logo on the wall.
Continue to follow Riley as she leads you to the mall. As soon as you drop down into the mall, head down the first escalator, turn right and go back the opposite way. Approach the Affordable Getaway poster on the wall, prompting another Optional Conversation. When the conversation ends, keep moving forward from where you came off the escalator. A short distance ahead there is a water gun poster and the next Optional Conversation.
Continue to follow Riley until you reach the bottom of the last escalator. She will turn around, allowing you to engage in another Optional Conversation. Tail Riley into the tent. Near the back, you can trigger the fifth Optional Conversation and perhaps quench your thirst. When you exit the tent, move forward a few feet and you'll prompt the sixth Optional Conversation. Push forward with Riley until she asks you to help her lift a beam. When it crashes to the ground, look to your right and behind you a few feet. Boost Riley over the door and head inside.
When Ellie puts on the wolf mask, she and Riley will talk about it, automatically triggering an Optional Conversation. Head to the back right of the store, inspecting the jar of eyeballs which will give you yet another Optional Conversation. When it's over, follow Riley around the shop until she puts on a Dracula mask, kicking off your third Optional Conversation since entering the store. Head to the middle of the store, putting on the witch mask. When you turn towards Riley, you can engage in one more Optional Conversation. Move to the outside aisle of the store -- on your left when you first entered -- and put on the Triple Phoenix mask, which automatically starts another Optional Conversation. There's still more to do at this location. Head to the front of the store where you'll find a magic 8-ball skull. Interact with it to trigger an Optional Conversation. There's a lot here, so every time you set the magic 8-ball down, pick it back up and keep asking it things until there are no more options. You may have to do this six or seven times, if not more.
It's time to head out of the shop. The exit is located along the same wall as the Triple Phoenix mask. As soon as you get outside, take Riley on in a brick throwing contest. Gather bricks from the ground around you and throw them through the windows of the red car below. If you break all your windows first, you will unlock the bronze Brick Master Trophy.
Once the competition is over, follow Riley through some double doors and down several flights of stairs. When you hit the bottom, instead of going straight, go left through a door and grab the Warning Note on a table. Catch up with Riley and flip the circuit breakers, restoring power to the building. Tail Riley just a bit further to trigger a cut scene and complete the second chapter.