Pretty much you will need to extract the vanilla textures from game to get an idea how the texture files work, then use Gimp or Photoshop to make the textures you need for that mesh. dds files for diffuse, normal and specular maps but I do not understand what goes where in the texture map, but I guess there must be a way of generating a blank texture map somehow? I played around with Blender and created a basic cube.
I'm an intermediate modder for Fallout 4 but I'm not great at texturing new 3D models, as I do not know what goes where - without a lot of testing in Photoshop (which could take forever) there's no real way to tell where to put the textures in the file.
