NOTE: This tutorial assumes you are rather familiar with Source SDK and is not for beginners. Please familiarize yourself with Source SDK and the Source engine before attempting to follow this tutorial. You've been warned.
NOTE: This tutorial assumes you have Lethal Stigma already installed on your computer. If you don't, please install the absolute latest version including any patches before proceeding with this tutorial, otherwise you will encounter serious problems.
To start, launch Source SDK. Change 'Engine Version:' to 'Source SDK 2007' using the drop-down box, then click 'Edit Game Configurations' under the 'Utilities' section.
NOTE: If you do not see 'Source SDK 2007' in the drop-down box, make sure you have Source SDK 2007 Base installed. If you don't, exit out of Source SDK and install it, launch it once, then start Source SDK again. You can be sure you have Source SDK 2007 Base installed if you have successfully run Lethal Stigma via Steam on the computer you are on.
A new window should appear. Click the 'Add' button and under 'Name,' put something like "Lethal Stigma." For 'Directory,' you will need to browse to your steamapps/SourceMods/lethalstigma folder of your Steam installation. In most cases, this will be something like "C:/Program Files/Steam/steamapps/SourceMods/lethalstigma" but each system is different, especially if you're using a 64-bit version of Windows or last installed Steam from scratch several years ago.
Anyhow, once that's done, press 'OK,' then 'OK' again. Restart Source SDK, make sure 'Engine Version:' is still set to 'Source SDK 2007' and 'Current Game:' is 'Lethal Stigma' (or whatever you put for 'Directory'), then fire up Hammer.
When Hammer has loaded completely, go to the toolbar and select 'Tools,' then 'Options.' Under 'Game Data files:' you will see an entry. Select it, then press the 'Remove' button as it is not needed by Lethal Stigma and may cause some problems. Then click the 'Add' button and browse to your steamapps/SourceMods/lethalstigma folder of your Steam installation again. There you will see a file called "lethalstigma.fgd." Select it, then press the 'Open' button. 'Game Data files:' should now have only one entry that ends in "lethalstigma." Assuming this is true, press the 'OK' button, close out of Hammer and restart Source SDK one last time.
You now have all you need to map for Lethal Stigma. Whenever you want to map for LS, just make sure 'Engine Version:' is still set to 'Source SDK 2007' and 'Current Game:' is 'Lethal Stigma' (or whatever you put for 'Directory') before launching Hammer.
Lethal Stigma comes with several new and edited entities and features. For details on what they are and how to use them, check out this thread on the forums of the official Lethal Stigma website.
If you have any questions, please post them in the comments section of this tutorial and a member of the development team will get back to you as soon as possible. I hope this tutorial was helpful. Thanks for choosing Lethal Stigma!