How can I customize the node/add form for a content type? How can I add some help text above the form used to create a node of a specific content type?
What is the best way to do it in Drupal 7?
On the content type edit/creation page there is a field labeled Explanation or submission guidelines with a help text of: 


  This text will be displayed at the top of the page when creating or
  editing content of this type.


Simply put your instructions for the content type form in that field and save your changes.  

Example: