Frets on Fire is a kind of game you can plug-and-play, yet to get it to work and look better you have to do some additional steps - something more than simply download it from http://fretsonfire.sourceforge.net .
1. Download Frets on Fire from this thread. Download either version 1.1.324 either 1.2.451 - 1.2.438 is full of unresolved bugs and is not supported by the forum. Install either of them.
2. Download RF-Mod - version 3.5, if you downloaded 1.1.324 or version 4.0 (which is still in development), if you downloaded 1.2.451 and install them into your FoF folder. It is a mod for fixing really many aspects of gameplay and makes the game run overall faster.
3. Download C-Mod from this thread (the most fresh link is always in Capo's signature) and install it only if you have 1.1.324. This mod fixes the appearance of the notes, the speed of the neck and some HOPOs in songs.
4. Customize your Frets on Fire by adding some modifications to it. Always remember to pick a proper modification for your Frets on Fire version. Some mods are universal and you can install them on both 1.1.324 an 1.2.451.