First, ensure that the Developer Console is enabled. To do this, go to the Options menu, click on the “Keyboard” tab, and then click the button labeled “Advanced.” Make sure the box next to “Enable Developer Console (~)” is checked.
Then, go into the game. When you are at the place you want to noclip, press the button above the tab key (should say and [font='Arial,Helvetica,sans-serif']~[/font]). The game should pause and the console window will appear. To turn on noclip, type in "sv_cheats 1" (without the quotes) and press enter. This turns on "cheats." Then type in "noclip" and it should tell you "noclip ON". Press the console key (
/~) to return to the game. You now float through the world ignoring collisions and gravity, i.e. you can go anywhere you want, even through walls.
When you want to return to normal movement, return to the console (`/~) and type in “noclip” again. It should tell you “noclip OFF”, and when you start playing again, you will walk normally.