Without going into too much detail, using configuration files in this manner is one of the basics and you can probably find a lot of help online or in the help files. Basically, go to Package Configurations, enable Configurations, and choose an Xml file for your config file. The Wizard kind of walks you through everything and helps you map you Config settings to your Variables.

http://vyaskn.tripod.com/sql_server_2005_making_ssis_packages_portable.htm