I'm sure others will suggest other ways but this is what I do:
1. if there is a similar helmet import it (including skin partition), if not import the imperial male head... or any other but that is the standard base head (including skin partition).
2. make your mesh - either editing exising helm or creating from scratch to fit the head
3. select the helm (each piece will have to be done individually if it is in multiple pieces)
4. go to weight paint mode
5. go to the editing panel (f9)
6. either select exisiting bone (vertex group, under links and material) named bip01 head or create a new vertex group and name it bip01 head.
7. paint all verts of the helm to 1 for that bone.
8. repeat steps 3 to 7 until done all seperate meshes if applicable
9. export to nif (including skin partition) - before exporting select all parts of the helm but NOT the skeleton.
10. you should be good to go - however you will want to create a ground mesh so before moving on:
ground mesh:
1. delete the skeleton (it will be named sceneroot (or similar) in 99% of cases)
2. select the helm (or if multiple meshes for the helm select each one individually and do steps 3 & 4 for each)
3. go into edit mode
4. go to scripts->hull-> select convex->select 0.100 for precision -> say okay
5. select all applicable helmet pieces and the hull(s) you made in steps 3-4
6. export to nif; in the collision options in the right hand side of the panel, select clutter, metal (if helm is metal if not select one of the other ones or edit later in nifskope), weapon body location: none, don't export skin partition
and now your really should be good to go... go test!
edit: to answer your specific questions:
1. duplicating the head to start with is perfectly fine... for many things the best, how good it is depends on the helm you're creating
2.extruding is fine (you are reweighting in my instructions anyways, also the UV map you will want totally different from the vanilla head so I have no idea why they would say that).
3. for a helmet no need to import a skeleton, the skeleton from a preexisting helmet or head (just the one bone bip01 head) will work fine.
4. what seperate a static mesh from a non-static: animation basically... for clothing/creatures rigging (weight paint/vertex weights) tells it how to animate when the skeleton animates, for other things morph animations, transform animations and for clutter havok is basically a form of animation - just one you don't have to do anything for (other than right settings on export).
Let us know if you got any more questions
Pacific Morrowind