It is actually quite easy to do. There are a number of programs that provide a gui for this task. In my case I tried numerous programs for hours and ran into problems with each one. What I did find that worked was to simply use ffmpeg and convert the video from the command prompt. It’s okay, the command prompt is your friend. There really are worse things than using the command prompt. So, now that we have got your fear of the command prompt out of the way, here is command that you will need:

ffmpeg -i input file output file.webm 

It is that simple. For the newbies out there, the input file is the path
and name of your file for example lets say the file is test.avi in your 
home/linuxnewbie folder, so your code would be something like: 

ffmpeg -i /home/linuxnewbie/test.avi test.webm 

Then you would go back to your /home/linuxnewbie directory and find the 
test.webm file.