What steps are necessary to shader an existing nif really depends on what already is present in that nif and what dds files are already present. For example, file-wise a Firaxis non-shadered leaderhead basically consists of one nif file and one dds file, which "paints" the nif model. A shadered leaderhead again is one nif file, but five dds files ("diff" - the paint, "norm/nrml" - the contours, "spec" - the general gloss, "env" - the environmental illumination pattern and "env_mask" - the glossness for the environmental pattern). Have a look at the already existing leaderheads to get a feeling how each of them should look like.
Furthermore, the nif file itself also differs. Obviously, the nodes of the polygone model are set up in a different way, but also the shadered nif has a number of NiPointLight nodes which create different illumination sources.
To get around setting all the light sources up from scratch, I normally use the shadered nif version of the model I rigged my Blender creation to, into which I
nifswap my model exported from Blender. Then I right-click on the "NiTriShape" parent node of the polygone data and go Mesh->Make Skin Partition. On the pop-up I change the second entry from 18 to 4 and press Ok. Then I make sure that in the Block Details "Has Shader" is set to yes, Shader Name to TLeaderheadShader_20 and Unknown Integer to -1. And of course I exchange the dds file names in the "NiTexturingProperty" node to the ones of the new model.
Once that is done and working, the NiPointLight nodes have to be adjusted to fit the new background, but that is something to go into once the leaderhead model itself is working. Hope the explanation thus far is already helpful.
