It’s hard to say exactly without knowing what kind of doll you’re working with!  If she’s porcelain or something, this is not my sphere of knowing, unfortunately.  

If you have a doll with a soft head like a Barbie that you want to reroot, there are several methods and approaches for doing it.   If you search ‘how to reroot a doll’, you’ll get several results.  This video for beginners covers the basic stuff for a first time/potential only time.  There are lots of methods and schools of thought about it, but this one I think is the most obvious cut and dry “I show the hair in the hole I want and keep going” method.  You can purchase doll hair online in a lot of places (don’t use d0llyhair.com, she’s unpleasant, unreliable, and a racist!  What a fun trifecta!), and it’ll all vary in quality, so you might find it helpful to find a doll with nice long hair second hand and steal her hair.  Then you already know what color and texture it is.  

If she had a wig that you need to replace, I’ll send this out into the tumblr void for someone who knows the best first time place to try that, because I’ve still never made a doll wig! Most methods I’ve seen for making them include a lot of glue, a lot of sewing, a lot of painstaking threading, or all three for max result, so I’m absolutely not the person in the know on this front.  

Let me know if this is even the right kind of doll, and if not, the powers of doll tumblr can send you where you need to go!

If people feel like explaining or recommending things they use/like/know onto this post for @ageekyreader to use for help, feel free!