@Miax - If you are talking about the tutorial in your sig, I went through that. In fact, that tutorial was instrumental in my creation of a new collision for the monorail in my mod (thanks). But I just did not see, or I did not get, the steps needed to remove and remake text on existing textures. Could you point me to the relevant section(s)/chapter(s)?
Chapter 3 is the one I highly recommend, as it will show you the basics on how to make your normal maps as well as the regular textures in the GIMP. Chapter 3 will get you into the GIMP and get you going on editing a texture.
Chapter 6 will show you the basics of NifSkope, but you don't need to go deep here - just install and setup Nifskope, then focus on section 6.5 to learn Where textures are stored for a particular model.
Chapter 7 then will show you how to add texture-sets to the game, and how to attach them to your objects successfully.
I recommend you start with something existing, and modify that (which is how I learned) - like a beer bottle. Pick a sign that is close to what you want, find it's texture (Section 6.5 on NifSkope), and edit that in the GIMP. This will give you some practice while allowing you to modify a few clutter items like beer bottles to suit your mod. You only need Nifskope to discover what the current texture of the model/item is, so you don't have to go deep into NifSkope.
Once you load the texture for the beer bottle into GIMP, note the size (512x512, 1024x1024, etc), and stick to that same size. Then modify the texture however you wish. You'll notice how the 3D model sides/faces are layed-out on the 2D texture (which is UV mapping), but you don't need to worry too much about that right now - just focus on modifying the Part of the texture that has the sign (or where the label would go on the beer), and try it out. Flat signs really are ideal as early-objects, as they are primarily 2D objects and thats much simpler to deal with.
Post all questions! I am not a GIMP master, but Artisten and others here are and I'm sure folks will pitch-in to get you going!
Cheers