The good news is that you're not an idiot; the bad news is that, though the instructions may be straightforward, until you've had some practise it's far from easy.
Before you begin, I strongly recommend that you read the readme belonging to the addon. Some people can be far more uppity about strangers tinkering with their products than others. If it's a recent release you could try emailing them to ask if they still have the original texture for you to learn with.
.paa and .pac textures are in a format created by BIS, hence you are unable to modify them until you convert them to another format. The tool for this is
PaNtool by DSchulle. This can preview the textures and save them in .tga (Targa) format.
Then you should be able to modify the texture with most photo-editing software; GIMP for instance. The tricky business is saving your work in a way that can be converted back to .paa/.pac. There are a number of programs that can do this and their requirements vary:
- paatool by feersum-endjinn has the most options and a fair chance of giving a great result
- PaNtool (as above) requires the textures to be in 24-bit colour and occasionally outdoes paatool
- TexView is the BIS program and has the fewest features.