Playing Layonara on NWN can be summarized by the following steps.
Purchase Neverwinter Nights Diamond from a retailer. One is available for $9.99 for immediate download from gog.com
Download and install Layonara's free custom addons (called hak paks) from the File section.
Read up on the lore of the world to think of a character and how they will fit in the world.
Submit your character for approval on the forums.
Play!
For a more detailed description for getting to know Layonara read the Beginner's Guide to Layonara