A : You only have to do this one time, the first time that you install the plugin. Afterwards, each time you add a post with a video, the sitemap will automatically generate and add the video to the sitemap.
This will check all the posts on your site to see if the sitemap can find any previous videos that it can index in the sitemap. If it does find videos, you will see a success message with a link to click and view the generated sitemap .
If however, the plugin didn't find any videos in your post in a suitable format to add to the sitemap, you will receive an semi-error message .
This is not a true error message, it just simply means you need to add some videos to your posts so they can be found by the sitemap, and added.The second kind of error you could receive (very rarely will this happen) is if the plugin is not able to create videositemap.xml file or write to the existing one due to permissions. In this case you will either see, a pretty error message saying that the file cannot be written to, with a textarea with some code in it. It will tell to create the file and paste the code in the file.