This guide is intended for the Steam version of Half-Life, post 25th Anniversary Update.
Half-Life mods add or modified content to the game to provide a new experience. A mod can be as simple as changing the sound of the player walking to sound like a quacking duck or as elaborate as a whole new game with custom maps, weapons, sounds, and stories that make the game feel new.
To get started, make sure Half-Life is downloaded. If it's been downloaded for a long while, it's probably worth hitting the “Verify integrity of game files” button in Properties > Installed Files tab. This will insure the files are not corrupted and in working order.
Usually, Half-Life is installed here.
C:\Program Files (x86)\Steam\steamapps\common\Half-LifeThe main Half-Life folder holds the exes, game folders, DLLs, and other files.
The valve folder is where the game files such as maps, sounds, models, etc are stored.
Special Textures
Clip - creates a clip plane or invisible wall on all four cliphulls that players cannot move through, but can shoot and shine the flashlight through. Clip brushes can not exist outside the world, so keep them inside of your map.
https://www.sourcemodding.com/
http://www.slackiller.com/tommy14/tutorials.htm
http://www.the303.org/tutorials/gold_texture.htm
https://gamebanana.com/tools/download/5391#FileInfo_650197