Ok, It's easier if you have 2 windows opened, one for your Mod and one for your Game.
On the first you see a mod for Peta Jensen, second is when you click that open till you see the "game" folder for that mod.
Last is the Games opening part, where you can start the game, have that in a second window.
Now you click the folder "game" in the Peta mod folder.
Then you go to the Game start window and click on an empty space to indicate to your PC that this is the place to paste whats in your copy clipboard.
Then on this empty space you Paste your copy.
Now it will overlap the "game" folder you have a copy of, over the "game" folder inside the Game Start window and create everything that isn't already in there from that copy of the mod.
If you click on the "game" folder inside that game start window, it's likely for some OS to paste your copy inside the "game" folder, which is the wrong way to do it, because then you have the path:
"game->game->mods"
where it should be:
"game->mods"
Therfore click on an EMPTY space before pasting!
The one who made the mod, made sure every path is inside the folder "game" that you copy, so everything will be installed correctly if you do it this way!
As I wrote before, some modders put a shorter path to their content, in that case see what the folder in their mod is called, navigate till you find a similar folder inside the Games game folder, would usually be "game->mods" and use the same overlap technique I just showed ya

On the Spreadsheet where we find most mods, is also info explaining it all.
The example mod here, you also notice the mod creator has placed info text. This text is very informative in how to add it, as well as that mod is made a special way so you can add the girl as either a girl or a mother, the info text explains how.
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