Adding a Youtube video to an existing song in the database is extremely easy. Here are the 5 steps.
1. Log in
2. Search for an artist
Preferably an artist you're very familiar with.
3. Spot a song that doesn't have a video yet
They are the ones without the Youtube icon.
4. Hit the "Choose Youtube video" button
5. Pick the right version
... after having listened and verified it is indeed the corresponding song!
You have now added a Youtube video to the database!'''