It is a very popular component with lots of downloads and a detailed documentation, simple to install and use:
- Download the latest Xmap version version here.
- Unzip it and install both files (com_xmap-x.x.x and plg_xmaplink)
- In the backend go to the Xmap configuration window at Components --> Xmap.
- It is easy to add a new menu or modify default priority options.
- Now go to Options --> Preferences and copy the Xml sitemap url.
- Click on Add Sitemap on Google Webmaster Tools dashboard
- Add the copied url and make it "fit", just copy what's after yoursite.com/ this should be something like index.php?option=com_xmap&sitemap=1&view=xml
- That's it!