Wet the hair with warm water. Use warm water but not HOT. Hot water can destroy some of the old hair fibers.
Use a little teensy dab of shampoo if you must (it doesn't rinse out very easily.) I prefer Spray 409 cleaner because it's an effective but mild cleaner and rinses easily.
If the hair is really grungy-gray and it needs bleach, then I use bleach cleaner spray but only if the hair doesn't come clean with anything else. Bleach is nasty on my hands and it's not so great for dolls either but sometimes nothing else will work.
Sometimes I rub cheap hand lotion into the wet hair to soften it up a little. And lotion will help to dissolve dirt and grime as well.
COMB AND CONDITION. Once the hair is clean and rinsed, then pat dry with a towel and spray on the hair conditioner. Use the metal tooth comb to comb the hair. Work from the ends first and patiently work up to the scalp gradually. This is the best way to conquer those awful tangles. Don't start from the scalp and work down. (If you pull out the hair it won't grow back.)
I have combed out Barbies with two feet of hair that was completely ratted, so I speak from experience: you can conquer the tangles and rescue the doll's own hair! Just remember to start with the ends and work your way up.
Once it's all combed out nicely, then comb into a nice style. If you need curlers, now is the time to add curlers or perm rods.
And finally; put a knee high nylon stocking over the head to hold the hair in place. These are cheap; you can get a package with several pair at the dollar stores. Knee-highs work for most medium size dolls. If you have teeny dolls you may want to use a doll sock to hold the hair down. Allow the hair to dry, which usually takes about 24 hours.
IF YOU NEED CURLS. You can curl the hair while it's still wet. Use small plastic perm rods, the kind with the rubber cap on them. Or small hair rollers, if it's a big doll. Or, we use straws cut into pieces for very small dolls such as Ginny or Barbie. Straws with bobby pins.
You can get small plastic perm rods at a beauty supply store and they're cheap. They're nice to have on hand for dolls. If the perm rods don't stay in the hair very well, I also use small 1 inch rubber bands to hold them in nice and snug. Small rubber bands are really handy if you work with dolls; get them at the beauty supply store, too.
Curling doll hair is not easy and requires a bit of fiddling. Once you've got the hair in curlers, then spritz each one with a little bit of hair spray or dab them with a protein based hair gel such as Crystal Ice. Protein based gel works better on doll hair than glue-y type hair gels.
If you use curlers, it may take a day or two before the hair is really bone dry. You want to wait until the doll hair has "submitted" to the curler and will retain the curl. Once everything is really dry, then comb the doll's hair again and add a ribbon or other trims if you like.
If the doll has mohair, you may need to leave it in the curlers for a couple of weeks, and warm up the hair with a hair dryer to help force the mohair to SUBMIT.
IS THE HAIR DRY? If the doll's hair is dry and brittle, you can use Avon's Skin So Soft on the hair, or baby oil. Don't use food oils, they will become gummy and sticky. Synthetic oils are good for synthetic doll hair because that's what it is made with, originally (Petroleum products.) Mineral oil, baby oil or Skin So Soft will restore the moisture to the hair. You can also use hair dressings such as pommade. I have very dry, dull hair so I often use my own hair products on my dolls.
Your doll will look much better than you could have believed was possible! And it's better to keep her own hair than to give her a wig. Wigs change the appearance too much.