Generating sitemaps and the Google Merchants Product Feed can be done in one click. Navigate to your dashboard and go to System > Sitemaps.

Once there, click the checkbox to include the Google Merchants Product Feed.

Finally, click Generate Sitemaps and both the sitemaps and the product feed will be generated.

The Google Merchants Product feed is accessible via your web store URL, e.g.

Note that, at this time, if there are changes to the products, sitemaps need to be regenerated for those changes to reflect.