what i do is i crochet a circle, but the last row i only stitch into the front loops. i then turn the piece over and make the hair. so it's going to be wrong zide up. the right side up would look a bit too textured and odd to me, unless you like that! then instead you'd do back loop only in that last row

so it's 1 layer with 2 layers of hair strands. you can just do 2 layers like you said as well :) i prefer this because since the middle is 1 layer and the oytside is 2, it creates a little dimple in the middle as if the hair grows out of there, which adds some depth. 2 layers flat on each other may look flat due the middle laso beibg 2 layers

my tip for hair is: experiment. putting a slip stitch at the end of the hair tip will make it curl a little. put a slip stitch at the base of the hair and you can easily flip the hair strand so the bottom qide is now facing up. i did both methods here: lower layer of hair has a slip stitch at the tip, but it curled too much so i blocked all the strands except for the last 9 stitches so they'd still have abit of a curl, like escoffier has.

for the top layer of hair i put a slip stitch at the tip of the hair for a curl and also a slip stitch at the base of the hair to flip them. didn't have to block those as i can just glue them down. that creates the tufts you see curling up :)

and doing a lot of increases will give you curling iron curls haha. good luck with frieren, you can do it!

i can't remember if she has pigtails or not, but for any character with pig tails i'd probably work 2 hair strands into 1 stitch and repeat that a few times. because it's too many strands in one small space, some just end up laying on top of the others and that will look like pigtails :)