This is used if you have multiple meshes that you want to combine into one hair. You don't need anything under 'Extra Parts'. You don't need it (I'll say more about this later). Under 'Model' select the NIF file for the hair. Look at the settings from one of the the other hairs as an example (E.g. You add the hair in the Creation Kit by creating a new head part in 'Character/Headpart/female/Head Part' for females or 'Character/Headpart/male/Head Part' for males. When editing the NIF in NifSkope, copy the settings from a vanilla hair or a mod hair instead of an armor. If you made the hair at ground level, like the way vanilla hairs are made, then use the skeleton from a vanilla hair to rig it. When rigging, if you made the hair at head height, like the way most mods do it, then use the skeleton from a mod. It's probably best to do it like that for now, until you get more experience. Most mods just use SBP_131_HAIR, which makes your character look bald when you equip a helmet. When it mentions assigning skin partitions (or dismemberment partitions), use BP_HEAD for parts of the hair that disappear when you equip a hat or helmet, then change it to SBP_131_HAIR in NifSkope, and use BP_HEAD2 for parts of the hair that stay visible when you equip a hat or helmet, then change it to SBP_141_LONGHAIR in NifSkope. You can follow Hana's armor tutorial to learn the process for making armor. (You might be able to do it with the most recent Blender, but if you want to use the method that definitely works, then use Blender 2.49.) Set it up according to the instructions on this page:

If you don't know how to use it then start with the tutorials here: You didn't mention if you knew how to use it or not.

You'll need to know the basics of using the Creation Kit. The process for making hair for Skyrim is pretty much the same as the process for making armor.

Skyrim any hair all races