Hello. I assume you are here because you are trying to play a Source Mod but don't wanna pay money for the required game. Well I'm here to tell you how!
So, first you want to download your mod. I'm going to use the mod Terminal 7 as an example. Next, you want to go to C:/Program Files (x86)/Steam/steamapps/sourcemods. Extract the mod folder into that directory. Next, open the mod folder and locate gameinfo.txt. In my case (and most other cases) it would be located at C:/Program Files (x86)/Steam/steamapps/sourcemods/(mod name). Open gameinfo.txt and locate the piece of text that says steamappid, followed by and id, and maybe after that the name of a software beginning with Source SDK. This is what we want. Once you have found what the software is called (you have to download the required one!) open Steam and hover your cursor over library. A dropdown menu should appear. Click on tools, and locate the required software and install it. Next, open it once it has downloaded, and if the option is there run a video stress test. If not, just exit out. Now, restart Steam and the mod should appear in your games library.
One more thing I want to mention. If you get an error saying "Could not load library client", go into the Source SDK's sourcetest/bin folder (not the mod folder!) and locate server.dll and client.dll. Once located, copy them and paste them into the mod's bin folder. Overwrite any requested files and restart Steam again. Have fun!
P.S. If you have any questions or something has gone wrong, leave a comment here or PM me.