Instructions
Start the game as you normally would until the glass windows in the doors have all been broken. You now need to construct barriers to prevent the zombies from overrunning you.
Walk up to the windows and press and hold the "X" button (square button on PS3) to construct barriers in place of the broken windows. Each barrier is made of six wood planks that will be placed one after the other while you are holding the button.
Monitor the barriers closely and repeat Step 2 as necessary to repair them. If you don't monitor and repair the barriers they will be broken down by zombies very quickly, so this is vital to your survival. As long as you keep up on the repairs your barriers will continue working.