So you've decided to use Tumblr for your blog, and you've decided to write posts in advance! This is great news, because auto-posting and Tumblr are both very easy.
As a warning, Tumblr sometimes suffers horrible glitches wherein it dumps your queue in one fell swoop. This is irritating, and it's happened at least twice since I jumped on the Tumblr Bandwagon with reckless abandon. With that warning out of the way...
Log in to Tumblr, go to your dashboard, and choose the type of post you want to make.
You can write your post first or later, it's up to you.
The column on the right is where you distinguish whether this is a draft or not. Click the drop down menu, and select PUBLISH ON.
Tell Tumblr when you want your article published. Fill out the other fields, too.
Click SCHEDULE POST. If this was a draft, the button would say save, and if it was going to publish now, it would say PUBLISH POST.
Now your article is scheduled to post in the future! Awesome!