Requested in the Map WIP thread. Maybe there’s an easier way to do this, I’m no expert, just posting what worked for me.
Step 1: Download Alien Swarm (It’s Free) and run the Alien Swarm SDK (Steam tools menu)
Step 2: Click Edit Game Configurations and then Add.
For Name write CSGO
For Directory put the full path to the Counter-Strike Global Offensive\csgo folder. For me the path was:
c:\program files (x86)\steam\steamapps\common\Counter-Strike Global Offensive\csgo
It may be different for you.
Click OK twice and restart the SDK.
Step 3: Set Current Game to CSGO and run Hammer.
Step 4: Go to Tools -> Options
Make sure Configuration is set to CSGO
Click Add and browse to Steam\steamapps**[your steam account name]**\sourcesdk\bin\orangebox\bin and select cstrike.fgd
Replace Game Executable Directory with $SteamUserDir\Counter-Strike Global Offensive
Replace Game Directory with $SteamUserDir\Counter-Strike Global Offensive\csgo
Replace Hammer VMF Directory with $SteamDir\steamapps**[your steam account name]**\sourcesdk_content\cstrike\mapsrc
Click OK and restart Hammer.
Step 5: Start a new map and save it. You must put .vmf on the end of the filename or it won’t save properly.
Run the map. If you get the normal compile window click Expert.
Select Default configuration
Click $bsp_exe and remove -alldetail from parameters
Tick the first four boxes
If everything went well you should now be able to use CSGO textures and models and any CSS entities that haven’t changed.
Run compiled maps by running CSGO and typing map mapname in the console.
One final thing I forgot: When you create a new map you have to change the skybox to sky_dust or any other CSGO skybox you can guess the name of.