How can I order posts by custom field value

I am trying to set up a blog and within each category I want the posts sorted by the value in a custom (postmeta) field.

Each post will have a custom field called “Rating” which will be a numerical figure from 0-9. I want posts rated “9” to show at top and posts rated as “0” to appear at bottom.

What is the easiest way of achieving this ?

TIA

David