The thinner strands use less yarn than the whole 4 ply yarn and it looks nicer too. When you use yarn which is smaller than the yarn for the doll, you can use a smaller hook to get the yarn through the stitches. You can read how to insert the hair on the head on EssHaych blog.
|Sometimes I create a hair cap in the color of the hair. Do this when you cannot insert a lot of hair strands. The hair cap gives a full look and hides the bold spots. For the Pippi doll I could not use many strands: otherwise the braids would be very big. To hide the bold spots she has a red hair cap.|
(See photo left of the haircap for Pippi).
Instead of adding a hair cap you can also change the color of the head to the hair color. This I did with Jazzy the Mermaid and Nurse Jazzy. (This does not show on the photos below.)