For longish hair, wrap the yarn around a book a bunch of times. Anchor each yarn end to the book with tape. Slip a ribbon that matches either the doll head or the colour of the yarn under the wound hair. with tiny stitches, sew the yarn to the ribbon. When you're done sewing, slip the yarn off the book, and cut at opposite end of ribbon and stitching. Sew this to doll head. Repeat with more of these "wefts" in different areas of the doll's head to add thickness; this also helps to cover the doll's head.
You can use this technique for a short style, too but you can also cover the doll's head with turkey stitches. Just make the loops whatever length you wish:
Here is a great example of a craftster who used turkey work (I believe) to make this great doll!
`Hope this helps!!