For the purposes of this tutorial, there are two important directories you need to know about. The first is where the 2013 SDK is:

Steam\steamapps\common\Source SDK Base 2013 Multiplayer\

the second place is your BG3 directory:
Steam\steamapps\common\Battle Grounds III\bg3

In the future, I (and Hammer), will alias these as, respectively:
$SteamUserDir\Source SDK Base 2013 Multiplayer\
$SteamDir\steamapps\common\Battle Grounds III\bg3

First, in your Steam Library, click on Add a Game and do add a non-steam game. From there, navigate to the $SteamUserDir\Source SDK Base 2013 Multiplayer\bin
folder and choose hammer.exe .

This will add the 2013 hammer to your Steam library for easy access. If you want, right-click on it, go to properties, and rename the shortcut to Source 2013 Hammer.

Open Source 2013 Hammer (the one you just added). You'll be asked to choose a CFG. This doesn't matter because we're gonna make a new one for BG3, so just click ok or next. If you don't get this message, don't worry about it.

With Hammer open, go to Tools->Options. This is where most of our work will be done.

• With hammer options open choose "Edit" then "Add"
  
• Give your CFG name, ex. BG3 and press OK and then close "Edit Game Configurations" window
  
• From drop down list choose correct CFG (the one you just added)
  
  
• For Game Data files, you need to Add these two files:
  
  • $SteamUserDir\Source SDK Base 2013 Multiplayer\bin\base.fgd
    
  • $SteamDir\steamapps\common\Battle Grounds III\bg3\bg3.fgd
  
  
• I'd recommend making your default entity types match what I have in the picture, but it's not necessary.
  
  
• For the Game Executable directory:
  
  • $SteamUserDir\Source SDK Base 2013 Multiplayer\
  
  
• For the Game Directory, point it at your BG3 folder.
  
  • $SteamDir\steamapps\common\Battle Grounds III\bg3
  
  
• For the VMF directory:
  
  • $SteamDir\steamapps\common\Battle Grounds III\bg3\maps
  
  
  

Go to the build programs tab, as seen in the picture.

1. For Game Executable, it's:
   
   • $SteamUserDir\Source SDK Base 2013 Multiplayer\hl2.ex
   
   
2. For BSP, VIS, and RAD, it's respectively
   
   • $SteamUserDir\Source SDK Base 2013 Multiplayer\bin\vbsp.exe
     
   • $SteamUserDir\Source SDK Base 2013 Multiplayer\bin\vvis.exe
     
   • $SteamUserDir\Source SDK Base 2013 Multiplayer\bin\vrad.exe
   
   
3. For the last one:
   
   • $SteamDir\steamapps\common\Battle Grounds III\bg3\maps

Apply, restart hammer, check that you're using the right configuration from the options menu, and good luck!

don't port or make maps and compile them with the final names you'd want them to have. Suffix them with _dev1 or _beta1 or the like