Download the latest copy of redITEM from the Downloads section of the redCOMPONENT site.
Once you have the appropriate version, install the component as you would any other Joomla! extension; visit the Install/Uninstall page in 1.5 or the Extension Manager in 2.5, select the package using the "Browse" button, and click on Upload. A notification will appear to indicate that redITEM and the redITEM content plug-in have been installed successfully.
Open up redITEM by clicking on Components -> redITEM. The "redITEM" main menu screen will appear, displaying buttons and submenu links to the various sections of redITEM's backend. Also displayed is a "Statistics" module that provides an overview of the total number of items and categories with a breakdown of how many are published, as well as how many users have redITEM profiles filled in.