I've looked at a bunch of textures like the noblefurnchest and it has a lot of differently textured areas. How is a different part of the texture assigned to different parts of the model? If you only want a part of what shows when you open that texture with gimp, say the basic wood texture but not all the trim how would you go about it?

There is a UV map associated with each .nif file that determines which parts of the texture file are applied to the various parts of the model. You can see the UV map by going into Nifskope and right-clicking on the appropriate NiTriShape. Then choose 'Edit UV' and it will bring up a little window showing the how the UV map is distributed on the texture file.

OK. Got that far. Have a little box with lines and dots. But what do I do with it?

You can move the vertices around in that little window. You would need to position them to where the wood part is covered by the part of the model you want to show a wood texture. It's sort of meticulous and time consuming though.