I rescued a dog who was never leash trained. I am fearful that I will try the wrong technique and cause her to develop an even worse habit. What should I do?

We recently rescued a starved emaciated female pitbull from the side of the road. I have been watching her closely to pinpoint any aggression issues; but so far she has shown no aggression to our dogs that she has seen but not closely encountered or people. She is very submissive and shy although hasn't even really thought about biting anyone even when poked and prodded by the vets. Evidently she was never trained on a leash. This is fundamental stuff she needs to know ASAP. I have tried to be firm yet gentle with her placing a pinch collar on her and dragging her outside. She doesn't act fearful aggressive- just fearful and not certain of my intentions-even when I am giving her treats. I am fearful that I try the wrong technique I may cause her to develop an even worse habit. Since this is an unusual case I don't feel I can train her like any other normal dog. What can I do to make this process more comfortable for her but without compromising her viewing me as the pack leader?