Install the plugin from update site http://www.multimodus.hr/apex-editor-ls-update.
The plugin works with the Force.com IDE and Eclipse 3.5 (Galileo). Eclipse 3.4 is not supported. The Force.com IDE plugins are needed for this plugin to be useful.
In your existing Force.com project right-click any .cls or .trigger file and open it with the "Apex Editor LS". If you like this editor, you can edit the eclipse preferences and make it the default editor for .cls files.