Step 1: Pick yarn of your choice and wrap around a paperback book. Make sure the hair will be long enough to cover the head. To ensure head coverage the length of one full circle of the wrap must reach from chin to chin. By this I mean the length of the yarn wrapped once around the book must be at least long enough to go from one side of the doll's chin, over the head and to the other side of the doll's chin. If you want the doll to have long hair I suggest shoulder to shoulder.
Wrap the yarn around the book until the wrapped hair equals the distance from the doll's forehead to the nape of her neck. You want to have enough hair to cover the head from the forehead, over the top of the head and down the back of the head.
Step 2: Run a piece of scotch tape down the center of the wrapped yarn...horizontally based on the photo above. Then turn the book over and cut the yarn down the middle of the book. This will leave you with many individually cut pieces of hair held in place only by a piece of scotch tape.
Step 3: I recommend using a sewing machine for this task, but it can be done by hand...sew the hair together by sewing down the middle of the tape. Make sure the sticky side of the tape is down or it will stick on the sewing machine needle. Use thread a similar color to the hair to help it blend. The stitching gives your doll's hair a part of sorts.
Step 4: Place hair on the dolls head. Stitch down the part and attach to the sides of the head. Use the same yarn you used for the hair and it will blend right in. Tack down the hair on the sides and back of the head.
Step 5: Repeat the yarn wrapping process around a book, tape down the middle, cut it in half opposite the tape and stitch it together. Place the second layer of hair on the head and stitch in place at the part and as needed around the head.
Step 6: Cut the hair and style. For one doll I simply did a ponytail, and if you aren't good at hair styling I found this to be a very forgiving hair style.