If you want the new YouTube layout before it's rolled out officially you just have to follow this 4 simple steps:
1. Go to http://www.youtube.com
2. Press Ctrl + Shift and J in Chrome to open the Developer Tools
Press Ctrl+Shift+K in Firefox
3. Click on the "Console" Tab and enter
4. Reload the website and enjoy the new layout.
Some have problems with the browser chase so if you explore problems clear the browser chase memory. Feel free to share this post. @safegaard