Here's a tutorial:
http://s9.zetaboards.com/exilian/topic/467530/1/
And here's a tutorial on using AI mesh (not included in the other one):
http://forums.taleworlds.com/index.php/topic,61126.0.html
Notice it's fairly simple to learn, but time consuming, and especially castles can be difficult.
If you haven't done any scene editing yet, then start by editing a village or something simple.