First you must have the following to use LilyTool:
Java Runtime Environment 1.5.0 (aka Java 5) or later
You can download it from its official website.
jEdit 4.3pre16 or later
If you have this, run jEdit and choose >, select Install tab and check LilyPondTool. Click install and jEdit will automatically download and install all necessary files and plugins for you.
JEdit's Plugin Central doesn't always have the latest version of
LilyTool, because jEdit plugin repository is not updated so often, than
LilyPondTool. So you may like to download it from our project page.
Download the latest release of lilyj4edit
and unzip to the .jedit folder of your home
directory. You may also have to install additional plugins in this case,
jEdit will report the missin plugins upon starting.
The latest lilypond.xml mode file for jEdit syntax highlighting is also contained in the distribution downloadable from are project page, also from the jEdit SVN.