If you have a read of this thread here
, you should get a good idea of how appearance.2da works.
It's literally a case of opening the file up in KotOR tool and changing the model and texture names to whatever it is you want to use.
I don't have the games installed right now, so I can't really give you any screenshots or anything, but it really should become very clear once you've read the above mentioned thread.
As for the method I used, I added a new row to the file, then used a disguise property on the item itself to swap Carth's appearance to the new entry every time he equipped it.
I also added a feat to restrict the item to Carth. You would then need to use the TSL Patcher to make sure that the disguise property references the right appearance.2da row, but the TSL Patcher read me will tell you how that's done.