First you start the Tumblr Poster feature

You select the Link button.
On top you see the normal ninja settings for posts. Select the delay between posts, delay between reblogs and so on.
You can select where you want to add your message to.
Post Now, means in will be posted instantly, you can add your post to the draft or queue or post it privately
Click on LOAD BLOGS and all your tumblr blogs will be loaded into the list. You can now select the blogs you want to post to.
Add your title, url, description, author and you can add an url to a thumbnail into the form. Don’t forget the tags!
Click on ADD.
Give your project a name and SAVE your project
TUMBLR TUTORIAL VIDEOS:
How To Auto Post Links To Tumblr
How To Auto Post Your Links To Tumblr Queue
How To Auto Post Your Links To Tumblr Blogs Draft
How To Auto Post Your Links To Tumblr Privately