Make sure that your installation is up-to-date (see Online Update)
Download all installers (full client, fix packs, language packs, addons, ...) that you wish to use - ideally onto the computer from which you will create the upgrade (using them from a network resource will work, but slooooowly)
Make the installers available via either a UNC path, a mounted network drive, or a web (http(s)) server.
