These instructions assume that you have already installed some flavor of Eclipse. If you have not, Eclipse can be downloaded from http://www.eclipse.org/downloads/.
If you are already familiar with installing Eclipse plugins, you can just use the update site URL below for your version of Eclipse.
http://dl.google.com/eclipse/plugin/3.5
http://dl.google.com/eclipse/plugin/3.4
http://dl.google.com/eclipse/plugin/3.3
If you are new to Eclipse, the following detailed installation instructions should be useful.
Google Plugin for Eclipse 3.5 (Galileo).
Google Plugin for Eclipse 3.4 (Ganymede).
Google Plugin for Eclipse 3.3 (Europa).
Note: If you are having trouble installing from the update sites, please see this FAQ entry.