There are already several tools that give us the ability to create new stuff for The Sims 4. Based on the Create-A-Sim Demo that was released, several things, from recolors to new meshes for hair, clothes, etc., have already been added to the game.
While modding and sharing are common practices in the Simmers world, you should always take into account the potential risks that may derive from importing custom content in your game. It's the Simmers' choice to use / install custom content or not. It's the Simmers' responsibility to safeguard their property (PC and/or software, game).
In order to install the custom content you need to copy the .package files (uncompressed) in the Mods folder.
- For CAS Demo: D:\Documents\Electronic Arts\The Sims 4 Create-A-Sim Demo\Mods * and/or
- For the game itself: D:\Documents\Electronic Arts\The Sims 4\Mods *
You can organize your downloads in subfolders but only one level depth! Otherwise, neither CAS Demo nor the game will recognize the downloaded files and the custom content will not be loaded and available for you. That is the case with the Demo and that's the case with the game at least for now, it could change.
Consider it a good practice to remove all mods and custom content before updating your game.
Further to the above, in order to install Lots and Sims, you need to copy all the extracted files in the games tray:
- D:\Documents\Electronic Arts\The Sims 4\Tray *
There are more than one file for each lot and Sim. Unzip your download and place everything in the above mentioned folder.
*Please note that the path is indicative and depends on your system setup and your installation, i.e. it could be on your C:\ drive.