For the complete The Last of Us: Left Behind experience, read Prima's free walkthrough.
The final chapter is a mixture of present time and flashbacks with Riley. Follow her and give her a hand opening the garage door. Go left and grab the cart, using it to prop open the gate. Follow Riley, staying right on her tail. Fight off any infected that attack you, and melee any that attack her. Then, get back to following her as quickly as you can.
The rest of the chapter is just a cut scene showing the ending to the Left Behind DLC. We promised that we wouldn't spoil it for you, so we won't. Our walkthrough should have guided you through how to beat each chapter, have all Optional Conversations, final all Artifacts and unlock all Trophies. We'll leave the plot for you to discover on your own. Trust us, it's worth it.