How do you put mods in PC games

the way i saw it done for the game half life was that the deasighners made tools that they used to make the actual game and gave it for free to people to "modify"the tools are "tecture editors""3d charcter editor"level editore"sound editore""code editor"ect some companys sell mod tools that can be extended to many games.BUT a KEEN KNOWLEDGE OF PROGRAMING coding tecturing aect are ESSENTIAL>


i think you have to download another game onto your game that your trying to mod