Well, it says on that page how to get the multiplayer, so its fairly self explantory...
but here it is anyway. Or you can just look at the link.
Navigate to the directory containing Steam.exe Step 2
Then goto SteamApps/YourUserNameXXX/half-life 2/cfg Step 3
Create a new text file and name it servermode.cfg Step 4
In the file and the following
maxplayers (whatever you want)
Save the file and load up Half-life 2 Step 6
Open the console and type exec servermode.cfg Step 7
Now in the console type map <mapname of your choice> Step 8
When it loads up open console up and type restart
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