Dunno if this was from before last night but here’s a reminder anyway:
Log in to Tumblr, view your blog by going to it’s URL, click on “Customize” in the top-right, choose “Pages”, then “Add Page”.
Enter what you want the URL of the new page to be (http://blogname.tumblr.com/NAME) and keep the “Standard Layout”. Enter the actual page title and text.
Tick the box called “Show a link to this page”, then click “Create Page”.
Viola! You are done. You can edit the page by going to Customize>Pages and clicking “Edit” next to the page you want to change.
Pete