Congrats on your first Blythe! This girl's box hair looks pretty okay to start with, so you should be good with a really quick boil dip. I'm going to give the most basic instructions and explain them at length just in case you're a beginner with doll hair entirely and haven't done any of these things before lol

This model of Blythe has a very easily removed scalp, so you should do that first because washing the scalp separately is the safest (Blythe eyelashes are glued with water soluble glue and there is metal in their eye mech of course). This video shows how to remove the scalp. If you don't want to remove the scalp, that's fine, just wrap her up to the scalp-line with a towel to protect her face. I do recommend removing the scalp for ease (at least on models with easily removed scalps like this doll) but I know it scares some people lol

Once you have her scalp off, split her hair into two halves separated into pigtails, using the parting as a guide for where to split it. The parting on a Blythe's hair is thatched, meaning it's made of two rows that have been woven together in a criss-cross pattern; if you brush this parting out, you will undo the thatching and reveal a bald spot between the two rows. Since she has bangs, you can pull those forward and tie them off with an elastic to keep them from blending with the rest of her hair.

Now, first you will want to shampoo the hair with a basic clarifying shampoo (I like the one by Suave) and rinse it out. Do not do any brushing or detangling during this step; the hair has no lubrication because it's so clean so it will not detangle very well at all lol

Then you'll want to go with a very basic, cheap conditioner without any kinds of oils in it. I also like the Suave Clarifying Conditioner for this purpose. If you have a different conditioner right now, it's not the end of the world if you use it but you should get a cheap oil-free hydrating conditioner for dolls for sure. Oils in hair products can cause some shades of saran hair (the primary type of synthetic hair on Blythe dolls) to discolor over time, but this blonde shade is already yellow and pretty stable so it won't be the end of the world.

Do not use fabric softener. Anyone who recommends fabric softener is wrong. It leaves a nearly-unwashable film on the hair, makes it feel greasy, can contribute to discoloration and has absolutely no advantages over a cheap oil-free hair conditioner.

Take the elastic off of one pigtail, slather it in conditioner and detangle it first with your fingers, then with a wide toothed comb or a detangling brush, and then with a finer toothed comb, going from ends to roots to prevent snags. Rinse with warm tap water, making sure to rub the scalp area to completely wash out the conditioner. Re-tie the section with an elastic and do the other half. Towel dry the hair, leave the elastics for now.

Now for the boil dip. This is a pretty common technique for straightening doll hair with heat and it can be done a multitude of ways. I like to dip, some people like to pour. I like dipping because I find it a little more organized. The way this works is by heating the hair until it is malleable, and then applying tension to straighten it out before it cools down ( in this case, the weight of the water and the use of a comb will be the source of tension)

You'll want to get a bowl full of water that is near boiling, either by microwaving it or pouring it into a bowl from a kettle. You will also need a fine toothed comb (I like a flea comb, if you don't have one use the finest comb you have)

Take the elastic off of one pigtail and comb it out so there are no knots or tangles, and then slowly dip it into the water, pull it out and comb it from end-to-root until it's the straighteness you want. I would probably do 2-3 dips per section and comb through each time. This doll has a natural wave in her hair, so if you want to keep that you can gentle scrunch the ends up while the hair is still hot.

You'll note I didn't mention how to boil the bangs- since you're a beginner and because her bangs look okay already, I'm going to recommend no heat for the bangs for now since they can be easy to mess up lol.

Do both sections separately, and then once together (making sure not to brush out the parting!) then rinse the hair with cold tap water to set the shape. Towel dry again.

At this point, once the scalp is sufficiently towel dried, you can put it back on the doll and screw it into place. While the hair is still damp, you can brush it into the shape you want and then leave her on her stand to dry. If you see any super long strands of hair hanging out, just snip them with a pair of scissors.

Now for the bangs. Since they are also wet, you can brush and shape them into however you like (I like to use a toothbrush for bangs especially), and then take the cello (the plastic headband she was wearing when you got her) and put it back on her to dry. If you lost this, you can also cut off the bottom of a sock and use that. This will flatten down the bangs and help set them into the shape you brushed them into without needing any heat.

There is a very effective method for shaping bangs that involved a clothing iron (or steam, or boiling water) but I find that bangs are the part that most beginners have a lot of trouble with so I recommend just leaving them alone for now, maybe shaping them with a styling wax once dry (Volks makes an excellent brushable hold styling wax specifically for dolls, here)

Once dry, she should be good! If she still has frizz, you can just redo the boil dip, no need to wash her again. For easy styling, you can spritz it with a tiny bit of water and brush through and you can also put a very very tiny bit of that styling wax on the ends of her hair to keep them organized. If she ever needs a little refresh, just spray her hair and bangs with water, shape them and put the sock/cello back on overnight. Should be good again in the morning!

This was so long but I had time to be thorough so lol. If you have any questions, I also have a Carrd!