1. log in to a YouTube / Gmail account.
2. go to ... http://www.youtube.com/yt/copyright/copyright-complaint.html
3. Click on the "Submit a copyright complaint" button.
4. Complete the form which appears: I found you can get away with filling some fields with "N/A" ...
[ above form easier to read via this download ... http://www4.zippyshare.com/d/3284407/941195/template-32.png ]
Completing some of the form fields with "N/A" worked for me, e.g. I did not give YouTube my telephone number for them to take down the YouTube video which which entirely consisted of my Freesound which had been used without attribution, ( I had given the sound an attribution licence on Freesound ).