From Fedora Project Wiki
This procedure works regardless of where your Tumblr instance is hosted.
Creating your .planet file
.planetin the root of your home directory on fedorapeople.org. The contents of the file follow the pattern below. In the brackets [ ], use the URL for the category specific feed from the previous step.
[http://yourname.tumblr.com/tagged/fedora/rss] name = Your Name face = http://yourname.fedorapeople.org/yourpicture.png
Upload your .planet file to your FedoraPeople space
Tagging your post in Tumblr
Finally the only thing that you need for your post appear in the Planet is add the tag #fedora on your post.