For Windows users, the easiest way to convert the video will be to download the WebM DirectShow filters and use the makewebm tool inside the package.
To run the encoding process, simply run the makewebm in command line with the input video parameter:
It will start the process of encoding the video for you. If you receive an error about muxing audio or something like that, then probably you don’t have the Vorbis DirectShow filters installed. To do so, navigate to OGG page and download the codecs from their website.