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If you are working with models from 3rd party downloads, you may find that they're stored in a scale that is 1000x larger. This is commonly because they are working in millimeter scale for 3D printer software. In that case you will need to actually scale your models up or down before working them into NWN. Find how to do that using the help manuals for each 3D editor software.
Understanding Tileset Related Resources
To do all the work needed to build a custom tileset, you'll need to understand where the data comes from during gameplay. Below are some filetypes, specific files, and what they do.
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Tiles can make use of TGA and DDS textures. They can also make use of MTR material files, which can point to custom SHD shader files.
Understanding Tile Model Parts
Aurorabase
This is the specially dummy node underlying every model. It contains information about the tile name and supermodel linking, is the base for its tree structure, and contains the list of animations on the tile.
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Water texture has a few special options. The first and more common option is fancywater, which is started by using a TXI file, but uses a special shader. The second older method is to use a TXI file to run an Arturo Noise function on the water texture.
Understanding SET File Parts
Path Nodes
Path nodes help units move through the tile from another tile to another tile. These are used instead of the walkmesh on the tile, allowing distance pathing to see the tile as a single simple node in a route.
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Like doors, sounds can be added to tiles. This is not suggested.
Get to know your tools
You'll need various tools to build a tileset.
For making models, you'll need either Max/GMax or Blender.
To export NWN-ready models, you'll need NwMax or NeverBlender addon for that respective program.
To edit any model, 2DA or SET file by hand, you'll need a simple text editor, such as Notepad++.
To edit binary GFF file formats, such as to make a custom ITP file for your SET file, you'll need a GFF Editor.
To extract or view existing content locked in BIF files, you'll need NwExplorer or similar.
To extract material from custom content and to pack your own custom content for sharing, you will need a HAK editor.