1. First of all, go to YOUTUBE and choose the song you want. Copy the URL.
2. Go to THIS site and paste your URL where it says URL To Video:. You can change some things in your player, for example, make it invisible and with an autoplay - you just have to select the setting you want. Now click Make! and it will show you a preview!
3. Copy the code and then paste it in your description on Tumblr (Customize > Description).
Like this post if you agree! ;)