Choose first one of the vanilla hairlines and double click on it to open a window like the one shown in picture Add a new hairline. Hairs in Skyrim are composed of two headparts, hairline and hair.In the Object window expand the menu Character and then select the submenu HeadPart.
If you have the permission of the authour of the hair to redistribute it in your follower mod, or you don't want to redistribute your follower mod, then you can use this method to give your follower a custom hair: Note: If you want to use a face created using Expired's CharGen Extension mod, check this guide Using a face made with Chargen Extension on an NPC. To be able to do that change temporarily your mouse settings to work for the right hand and once you are done you can revert the mouse settings to the left hand. Note: The preview window doesn't work if your mouse is set for the left hand. In the Character Gen Morphs tab you can change also other characteristics of your NPC's face.
You can see the changes in the preview window (check the Head flag to see them).