Tutorial #000 - Installing and Configuring Hammer++ for Garry's Mod

1.1 Installing the requirements

To be able to use Hammer++ for Garry's Mod, you need to install the Source SDK 2013 Multiplayer tool
Make sure you have also installed Garry's Mod, Half-Life 2 and Counter-Strike: Source (for TTT mapping).

Meanwhile, you can download Hammer++ here and extract the folder to your desktop.
Once the SDK is installed, click on ⚙ > Manage... > Browse local files
On the now open Explorer tab, go to the bin folder and launch hammer.exe. Once fully open, close Hammer.

2.1 Installing Hammer++

On your Explorer tab, go back to the Source SDK Base 2013 Multiplayer folder and drag the bin folder that is in your extracted Hammer++ archive, into the folder.
You know if you did it right if you can see the hammerplusplus folder in Source SDK Base 2013 Multiplayer/bin.

2.2 Configuring H++ for GMod & TTT.

Now head into the bin folder and open hammerplusplus.exe.
Go in Tools > Options... and next to the Game Data files, click Add and add GarrysMod/bin/garrysmod.fgd and GarrysMod/garrysmod/gamemodes/terrortown/ttt.fgd.
Now get in the 2D views panel and make sure all the boxes on the right side of Options are checked.

Then, in the Build Programs panel, change the game executable to GarrysMod/hl2.exe and the compiled maps directory to GarrysMod/garrysmod/maps.
Finally in the Hammer++ panel, clicki the Anisotropic Filtering dropdown menu and set it to Off.

Close the Options window by clicking on Ok, go to File > New (Ctrl+N) then File > Run Map... (F9).

Click on Expert and select the LDR + HDR Full Compile -final configuration and click on the last command. On the right side click on Cmds > Exectuable and select GarrysMod/hl2.exe for parameters, put this string: -noworkshop -dev -console -allowdebug -hijack +map $file.

Now click the Edit button on the left side, click the LDR + HDR Full Compile -final configuration then Copy. Name it something TTT related then click on Close once renamed.
Select your new config and set this string for your last commands' parameters: -noworkshop -dev -console -allowdebug -hijack -gamemode terrortown +map $file.

Click on Close and then close Hammer++. On your Explorer window, go to Source SDK Base 2013 Multiplayer/hl2 and open gameinfo.txt.
Then replace its contents with the ones here. (You are free to comment out the games listed after CS:S.)

3.1 Making a shortcut on Steam

To make Hammer++ easier to access, you can add it as a "Non-Steam game" in your library.
On the bottom left of your library tab, click Add a Game > Add a Non-Steam Game... > Browse and select bin/hammerplusplus.exe then Add Selected Programs. You are free to customise your shortcut by renaming it and adding images from SteamGridDB

4.1 Conclusion

I hope this tutorial helped you set up your first steps into mapping with Hammer++! You can check other tutorials, here