Tutorial by pixelsgalore.tumblr.com

1: Paste the following code in your description:

<iframe frameborder="0" border="0" scrolling="yes" width="200" height="190" allowtransparency="true" src="http://www.tumblr.com/ask_form/USERNAME.tumblr.com" style="background-color:transparent; overflow:hidden;"></iframe>

2: Change USERNAME in the code to your tumblr url, for example I would change it to pixelsgalore