How can i get my tiel comfortable with me giving her scritches?

I've had Ziggy for just over 3 weeks. she is really comfortable with stepping onto me and loves spending time with me. She will happily sit on my arms and shoulders for long durations. She even sometimes preens my hair. I think we've bonded really well but she bites my finger if I try to pet her and I don't know how to progress with taming from here. I got her from a pet shop at supposedly 2 months old and she was raised by other cockatiels so she wasn't accustomed to humans and hands. How can i get her used to being petted?