Advanced Guide:
Apart from skins, the only other thing that goes into GameData/Skins is animations and animation meshes/objects. If you see BMP, CMX, SKN, CFP, BCF, or BMF files in the mod, then these go in the GameData/Skins folder.
BMP - texture
CMX - skin construct text file
SKN - 3D mesh
CFP - animation
BCF - binary compiled CMX file
BMF - binary compiled mesh (SKN) file
Objects (IFFs) that use custom animations will have these.
***
XA, WAV, MP3, HIT, HOT, and HSM files are sound related files. Leave them with the object or where they were meant to be installed. They have a fixed path, and if you move them somewhere else or rename them, the custom sounds for the object will be missing/broken.
If you are not sure where the XA/WAV/MP3 files need to go, open the HOT file in a simple text editor, like Notepad, and you'll see the path (you can also change it here).
***
IFF files contain the objects themselves. You can open and edit them in an editor like IFF Pencil 2, or using the Transmogrifier. Everything about the object(s) is contained in this file -- including the categorisation, description, price, and the object graphics, as well as the object's actions/programming.
Depeding on whether the object is overwriting an existing object, or it is a new object, it will go in a different place. New objects go in the Downloads folder. IFF files that are meant to override a game default object go either in the appropriate ExpansionPack folder, or in GameData/Objects, or GameData/UserObjects.
FAR files are archive files (like a
zip file) that The Sims can read without having to extract them. You can use a FAR extractor like FAR Edit, or FAR Out to see/extract the files in them if you want to edit the mod/object. But it's best to leave them as they are if you do not need to edit the mod/object.
When an IFF and a FAR file are next to each other, the IFF file overrides the IFF file by the same name inside the FAR archive.