1. Find a location: Player houses are characterized by a sign with a title such as “House #5”, next to a door and the sign should say “For sale!” with the details on how the interior can be designed, for example:
   
2. Go the the realtor: There's one for most islands or you can cheat and use the one in the OOC which is for all islands:

The interior of you house can be built using the Neverwinter Nights Toolset.

There's a few restrictions, they can be found on the page where you can upload your areas once they are done: https://nwn.sinfar.net/houses

Here's what the process looks like: