1. Log in as admin.
2. Go to the Plugins section.
3. Activate the "Configurable Tag Cloud".
4. Go to -> Appearance -> Widgets.
5. Drag the "CTC" entry from the left to the widget menu at the right to your liking.
6. Configure the settings.
7. Save and close it.
8. Check the result.

More information on the "Configurable Tag Cloud" plugin is here : http://wordpress.org/extend/plugins/configurable-tag-cloud-widget