How to install:
Download the .rar archive and use a tool such as 7-zip for Windows or The Unarchiver for MacOSX to extract the folders.
Copy those folders to:
Win Vista/Windows 7: C:\Users\<username>\Documents\Sports Interactive\Football Manager 2012\graphics\kits\
Win 2000/XP: C:\Documents and Settings\<username>\My Documents\Sports Interactive\Football Manager 2012\graphics\kits\
Win 98/ME: C:\My Documents\Sports Interactive\Football Manager 2012\graphics\kits\
Mac OS X: /Users/<username>/Documents/Sports Interactive/Football Manager 2012/graphics/kits/
Create folders "graphics" and "kits" if they don't exist already.
Then open Football Manager 2012, go to Preferences > Display and Sound and untick use skin cache and tick always reload skin on confirm.
Thanks to Madman99 at SortItOutSI for creating those kits!