Start out by launching Alien Swarm, and from the main menu press the "~" key (this is the " ` " key on UK keyboards, thanks to Ryry for notifying me) to open the console. In the console, type (Without quotation marks)

• "map asi-jac1-landingbay_01"

(Or any of these other map names):

• asi-jac1-landingbay_02
  
• asi-jac2-deima
  
• asi-jac3-rydberg
  
• asi-jac4-residential
  
• asi-jac6-sewerjunction
  
• asi-jac7-timorstation

This will launch a personal listen server to play on... I don't know if other friends can connect to this. In the room you've created, on the map landing bay... Select the tech marine you'd like to use for the map. Select your weapons, set the game settings (difficulty, friendly fire, etc) and start the game. Once the game has started, open the console again with the "~" key (If this does not bring open the command console, go into your settings and enable the developer console) and type

• "sv_cheats 1"

This will enable cheats on the server (Also disabling XP gain and achievement completion). After that, type

• "firstperson"
  
• "asw_controls 0"
  
• "asw_hide_marine 1"

This will set your view to first person, your controls to fit the first person viewing (more like an fps), and hide your character. You'll now be able to see the gun and laser pointer from the gun, and the view will be first person! Have fun there :)

Here's what it should look like at the end:

The only glitch so far with first person playing is during hacks. You'll need to disable the firstperson view and fps controls to do hacks... So I recommend just bringing a chainsaw for doors. To disable them, though, type

• "thirdperson"
  
• "asw_controls 1"

This will allow you to do the hack. Just set those two variables back to firstperson and controls 0 to continue playing in first person view :)

Some other downsides to the first person view:

• You're unable to see around you... just in front of course.
  
• Certain special items won't be able to be used... such as jet packs