How to install the Clinched skin on FM18
Download the .7z archive and use a tool such as 7-zip for Windows or The Unarchiver for MacOSX to extract the folders.

Move the extracted folder "CLINCHED18" to:
\Documents\Sports Interactive\Football Manager 2018\skins
Create folder "skins" if it doesn't exist already.

Start the game and go to Preferences screen, and then to the Interface tab.
You should see "CLINCHED18" as option in the skin drop down of the Overview box.
Use the Clear Cache button, then hit the Confirm button.

http://www.mediafire.com/file/bt4rty.../CLINCHED18.7z

Thanks to the superb work of the following:
Jovovich from fmnation
Gunzo Skin
Flut
DazS8
Andromeda