How do you use itumblr?

You mean “How Do I Use Tumblr ? ”

First of all read the documentation for creating a custom tumblr theme. You need to know both css and xhtml to design one custom theme.

Once you have come up with your theme,
1. Go to the tumblr dashboard
2. Click Customize
3. Click the Theme tab
4. Click Use Custom HTML
5. Paste your theme code

Alternatively, there is an awesome collection of free tumblr themes. You can find all of them at Custom Themes