I apologize in advance if the answer to my question can be found in a tutorial, but I haven't been able to find it in any of the tutorials I read.

What I want to do is add a new character. I have reskinned the twilek_06 head for TSL and have added new entries for appearance and head 2das. For appearance I just used one from a twilek female commoner and added my new head. I also copied the mdl and mdx files from the twilek female head, renamed them to correspond to my new texture and put all the files in the override folder. But I get the feeling that isn't quite enough... how exactly do I tell the game which model to use for my new texture? Where is this defined? And how can I get the appearance shown ingame for an easy test? I tried changing the appearance member of a party member using the KSE savegame editor, but that didn't work; the appearance stayed the same (which also happened when I tried changing the appearance to something else that was already in the game).

I'd appreciate any help.

As far as I remember, from recently making a new appearance & head:
1) You tell the game which model to use for your new texture by means of the "head" column in heads.2da
2) This is defined in the first column you should have made for your line in said .2da file.
3) Erm...not entirely sure what you mean by this question.

Hope this helps you in your quest for creating a new character.

-DB

Thanks for your help.

I now face a different problem though: I can't get the npc to show up ingame. I have used Kotor Tool's module editor to add my npc and then had it create a *.mod file which I put into the modules folder. But when I warp to the area, my new npc isn't there. I even tried starting a new game, but that didn't help either.

Any ideas as to what I might have done wrong?

There are 2 tutorial sections that should be considered priceless for you Hive.

The first is the 2da editing tutorial where there is at least 2 tutorials that would be 100% useful for the questions in your first set of questions one is by Stoffe and it explains the appearance.2da file it's columns and what they are used for. The other is by myself which walks you through from beginning to end the process of creating new playable characters in the game. The elements in the tutorial I provide are 100% useful when creating a non-recruitable NPC in the game by omitting anything involved in changing the portraits.2da file. For Recruitable NPC creation you will have to tweak the portraits.2da file for it to be right and proper.

Now the 2nd section that should be your friend and companion is the scripting section. As for adding your NPC in game should be handled through scripting. I would avoid using the module editor process untill you are completely ready to build your own areas. Even then I would still rely upon testing with scripting before hard placing a character into a module through the module editor.

Also the general tutorials section has the original getting started tutorials thread that has links to old threads that walk through many of the other various aspects of NPC creation and scripting to place your NPC in game.

Here are those 3 subforums.
General Tutorials -> http://www.lucasforums.com/forumdisplay.php?f=592)
2da editing -> http://www.lucasforums.com/forumdisplay.php?f=596)
Scripting -> http://www.lucasforums.com/forumdisplay.php?f=597)

Here are some useful threads from those subforums.
Appearance.2da Walkthrough -> http://www.lucasforums.com/showthread.php?t=148638)
Creating a New Appearance -> http://www.lucasforums.com/showthread.php?t=149992) (It has pictures)
Spawning items, creatures, placeables -> http://www.lucasforums.com/showthread.php?t=143536)
Do you want to mod SW-KOTOR -> http://www.lucasforums.com/showthread.php?t=151013)

Thanks but I did actually check out those tutorials already, and I was using one of the scripting tutorials before - but kept getting an error when compiling, so decided to try the module editor instead (which I simply couldn't get to work no matter what, and thus have abandoned again).

However, I resolved my scripting problem earlier (it was due to me not having paid enough attention to the error message received, no surprise really) and actually got it to work. Pretty neat. :)