Dear Josefa,
I'm sure lots of craft friends will post later about exactly how to do the ringlet curls and so on. Since I don't make amigurumi, I can't contribute anything about that. But I can cheer you all up a little, or maybe inspire you, with my post about doll hair.
I have lots of old B. dolls, which I also like to buy secondhand. Most of the time the hair is very straw-like, sticky, dusty, etc., and needs a beauty treatment.
That's why, after they arrive at my place, the first thing they always get is a beauty treatment for their hair. I put them headfirst into a container of almost-hot water with fabric softener added.
After about 30 minutes, they come back out. Then dry them in a microfiber towel (don't rub). Now comb with a detangling brush (from the drugstore)—first just the ends, then gradually farther up.
After that, blow-dry, but always only in one direction. Afterward, the hair is smooth, pleasantly scented, and shiny again.
And I've also tried out
curly hairstyles . Since it's not real hair but synthetic fiber, it's not all that easy. You wrap fine strands around cut-to-size drinking straws and secure them with hair clips. Then place them headfirst into the fabric-softener solution as above.
Then take them out, lay the head in a microfiber towel, and gently press out the moisture. Now blow-dry the hair carefully with low airflow, or put it outside in the sun. Then twist out the curlers and style the hairdo gently. If possible, don't comb too much, otherwise you'll quickly comb the curls right out again. They're not natural curls, after all :-)
Here you can see the two ladies after their appointment at my hair salon:
Best wishes from Ina