Hi,
DL the mod.
Most will have a readme that explains how to do it - but basically if the mod has a "Game" folder, then everything in it goes in the DDLC "Game" folder on your computer. The Scripts file and others usually get overwritten.
You also need to delete the *firstrun* file any time you want to start a game or mod from scratch.
Then when you launch the game, it will ask if you want to continue or start a new game. Select start a new game.
What mod are you gonna play?