1. Enter the Steam control panel then select the "My games" tab.
2. Right Click the "Left 4 Dead 2 Demo" option then select "Properties".
3. Select "Set Launch options" then enter -console and accept it.
4. Start the game, and the console window should now appear.
5. Enter bind p toggleconsole as a console command. Click and close the
console window and start a single player game.
6. Press "P" during game play to display the console window, then enter
sv_cheats 1 to enable cheat mode.