You will need to install a user script manager extension to install this script.
The latest version of the Controls for YouTube extension now includes an option to change the buttons to double-arrows, so you no longer have to keep this userscript installed. First, make sure you have updated to the latest version (1.32 or higher). Follow one of the links below to go to the page if you need. Then open the extension's options page and change the Button Style to "Rectangular Buttons" (picture below). You may as well uninstall this userstyle, because it's not going to do anything anymore.
Controls for YouTube is a Google Chrome extension which adds... controls for YouTube. Specifically, it adds fast-forward and rewind buttons over by the other controls. I made this userstyle so that those buttons more closely match the original YouTube interface. You MUST have that extension installed in order for this to do anything. I am not the author of that extension, so I cannot help you if you're having issues with it.
Install the extension from here:
Make sure that the buttons are enabled in the extension's config screen. I forget if they're on or off by default.