Making hair out of yarn is not hard. You can use whole strands of fine yarn for straight hair, or you can unwind thicker yarn for curly hair. The yarn hair can be stitched or glued onto a ribbon or gauze and then glued to the head, or it can be stitched straight onto the head if it's made from soft material. The simplest way is to make a thick hank of yarn and tie it together in the middle with a thread of the same yarn. Fasten this to the doll's head, gather together and make a bun or a braid at the back, pulling the strands together to cover the top of the head.

Crafty College has sometimes had courses in doll hair making: http://www.craftycollege.com