I actually had (still have, but they are 5+ now! :) ) 2 yearlings fillies like you described.
The 1st one was unhandled, but she was not afraid of people (that's a very VERY big thing in fact). So after 1 week of keeping her by herself (to be sure she's not sick) in big grass ring with lots of room I just let her go to other horses. And she always came to me, so I never had issues, I also didn't keep halter on her.
My 2nd one was wild, abused, and totally terrified of people (especially men, she still doesn't like working with males much). I guess it may sound mean, but I kept her in stall for one week just letting her run for couple hours/day in big round pen with short rope attached so I could catch her. I fed her myself every single day and while she was eating (she didn't even what grain is in beginning) I was standing by talking to her. I also try to communicate with her as much as possible and started brushing and all couple days after her arrival. Week later I let her go with other horses with halter. Another week later I took halter off completely.
It's lots of work, so be ready for that, but it's rewarding too! Good luck!