1. download this package, extract it and upload it to the doc root of your shop installation
2. open the file with a text editor or your IDE and enter your details for URL and database
3. fire up your browser at [http://www.youroxideshop.com/google_sitemap_xml.php] to generate a new sitemap file

Thanks to OXID forum users DIATOM for the basic script and to nullzehn, Piengie and uppaffrath for their modifications. This is the path to the appropriate thread at OXID Community forums: http://www.oxid-esales.com/forum/showthread.php?t=1933