TLDR version: Let her set the pace, practice patience.
Depending on whether she's a stray or semi-feral will heavily influence how long this takes. With a stray expect 1-3 months of patience to be required, with a semi-feral it can take 1-6 months (or more with especially fearful cats).
Now for how to proceed: First and foremost let her set the pace for things, do not corner her, and if she shows signs of fear: back off. As long as she eats and drinks, then things are moving in the right direction.
Starting off you might want to make sure you don't startle her, don't approach her suddenly without warning, but make sure you interact to some degree. To start with, talk with her, blink slowly at her. Then as she becomes more relaxed, push her a little bit about getting a tiny bit closer than the previous day, but not to the point that she shows sign of fear or hostility, and never corner her.
Eventually she'll become more and more relaxed, and at some point you'll be able to offer her your hand to smell (keep the hand near the ground, and try not to hover over her). Don't immediately go for a scratch, but make the hand offering a regular thing for a while before trying to give her a short scratch.
This might take a long time, so just be patient.
My story: I am currently socializing a 20 months old colony cat that I adopted 2 months ago. It's been slow going, the first month he spent in a large cage, getting used to my other cat and I being around. For the last month he's had access to the entire apartment. In the beginning he was so afraid that he'd hide in his cat carrier for the entire day, and only come out to eat and drink at night. Slowly he'd start to spend a few minutes sitting on top of the carrier, watching us, then trying to interact with my other cat.
When he was let out he'd hide whenever I so much as moved, but now he only runs if I start walking in his direction. He's even gone so far as to walk on top of the bed covers when I'm going to sleep, to interact with my other cats who he has come to absolutely adore. I've gotten to the point that he will smell my hand without running away, but he's not quite ready to let me touch him, and any touch will lead him to run away.