Step 1
Download the skin and extract the file (using either 7-Zip for Windows or The Unarchiver for Mac).
Step 2
Move the extracted folder "fmc2013fm" into your skins folder:
Win Vista/7/8: C:\Users\<username>\Documents\Sports Interactive\Football Manager 2014\skins
Win XP: C:\Documents and Settings\<username>\My Documents\Sports Interactive\Football Manager 2014\skins
Mac OS X: /Users/<username>/Documents/Sports Interactive/Football Manager 2014/skins
Create the folder "skins" if it doesn't exist.
Step 3
Start the game and go to Preferences screen and Interface tab.
You should see "FMC for FM 2014" as option in the skin drop down of the Overview box.
Hit the Confirm button.