This Kata was originally created by Terry Hughes (http://twitter.com/TerryHughes). It was then translated into many programming languages (including the Java version used here) by Emily Bache (https://github.com/emilybache/GildedRose-Refactoring-Kata).
I have also included a set of tests written by Sandra Parsick (https://github.com/sparsick) which can be used to ensure that nothing is broken while doing the refactoring exercise.
The "Gilded Rose Requirements" explains what the code is for. These requirements are also included as comments in the "GildedRoseTest.java" class.
The simplest way is to just clone the code and start hacking away to improve the code. The idea is not to re-write the code from scratch, but rather to practice taking small refactoring steps, running the tests often, and incrementally improving the code.