I am going to assume that you have a blend with the mesh and that's it, so if you have more than that then some of these will be steps that you have already done.
First of all, your mesh needs to be UV mapped. Don't use materials. FNV requires UV mapping for textures.
Second, make sure that you are using Blender 2.49b. If you are using a newer version of Blender, you can probably make statics with it, but anything requiring rigging isn't going to work very well. If you want things to actually work with FNV, you need to use Blender 2.49b.
You can find the correct version of Blender here. It has all of the nif tools that you'll need.
If you used a newer version of Blender and haven't rigged the mesh yet, my advice is to export your mesh as an obj and then import it into 2.49b as an obj. Exporting it from a newer version of Blender as a .blend doesn't work.
Ok, so at this point let's assume that you have your mesh in Blender 2.49b and that all you have is the armor mesh. The way that armor typically works in FNV is that it includes the human body parts underneath. If you have something that completely covers the human body like power armor, then you don't need the human body underneath. If parts of the body are visible, then you need to include the human body underneath with your mesh. Let's take the extreme example of the typical fantasy bikini chain mail armor. Most of what you see is human skin, not the armor, so basically the entire human body needs to be in the mesh. If you have something in between, like say an armored chest piece that covers the chest and back and includes a skirt of some sort, then you only need the human arms and legs. You can include the entire human body mesh, but most outfits don't since it wastes some time in the game engine drawing polygons that all get covered up by the armor mesh anyway, and if the armor mesh fits too tightly then sometimes you can end up with the human body parts clipping through, which looks bad.
So the first step is to import the female upper body mesh, as this includes both the skeleton and the human body parts.
Once that is done, remove any human body parts that aren't visible. Again, if it's something like power armor that completely covers the body, then you can remove the entire human body mesh. If it's bikini chain mail or something similarly revealing, you may want to leave the entire body there.
If you created the armor separately and didn't have a body to mold it around, you may want to adjust your mesh now so that it fits better over the human body.
Now, parent your armor mesh to the human skeleton (armature). You have a few different options for the bone weights. You can have Blender create the bone weights when you parent the mesh, which works reasonably well, but Blender tends to screw up a few things. Once you get some experience with it you'll be able to predict what parts will get screwed up and you'll have to manually fix them afterwards. Another option is to copy the bone weights from the human body mesh. In my experience this works pretty well for tight-fitting armor and clothing, but fails miserably when the armor or clothing isn't so tight-fitting. The third option is to paint all of the weights manually, which works better for solid armor pieces like a chest plate but doesn't work so well for fabrics and such that are supposed to move and deform with the body.
Once you have the bone weights done, save your blend so that you can always get back to this point. Then export your nif, making sure to use the proper settings. You can get the proper settings by clicking Restore Default Settings for Selected Game on the nif export screen, then go up to the top and click on Creature and then Skin under Collision Options, then make sure Use BSFadeNodeRoot is de-selected.
At this point you now have a mesh that is completely invisible in-game. This is due to Blender never setting the shader flags correctly. You fix these with NifSkope. For regular armor parts, the type needs to be set to SHADER_DEFAULT and make sure SF_SHADOW_MAP and SF_REMAPABLE_TEXTURES are both checked. For the any visible human skin body parts (arms, legs, etc, or the entire body mesh if you included it) the type needs to be set to SHADER_SKIN and make sure SF_SHADOW_MAP and SF_FACEGEN are both checked. Blender usually sets the rest of the flags correctly. If your armor has multiple meshes, you will need to fix these flags in every mesh part.
Create your new armor in the GECK, and you're done.
So basically, the short version:
1. Create your mesh
2. UV map your mesh
3. Import the human body mesh and skeleton
4. Remove any human body parts that aren't visible
5. Parent your mesh to the skeleton
6. Fix the bone weights
7. Export to nif
8. Fix the shader flags in NifSkope
If you get stuck, post here with details about exactly what you did and exactly what isn't working and we'll help you sort it out.