The UI is based around 1920x1080 resolution although it works with most resolutions.
1. Make a backup of your WTF, Interface & Fonts (if you have one) folders.
2. Download and extract my UI to your desktop.
3. Put my Fonts and Interface folder in the place where your own folders used to be.
4. When installing for the first time, make sure you select "Load out of date Addons".