Don't have her touch the gas pedal at all!
Stop the car in neutral on a dead-level parking lot. Swap seats, have her press in the clutch, put the car in gear, and then slowly let off the clutch. She might kill it the first time, but that's fine. After a couple tries, you'll get the car rolling slightly. This will teach her to feel for the clutch biting, and she'll learn the basics of clutch modulation. Once she can get the car rolling from an idle, THEN introduce the throttle.
Also, borrow a Miata if you can. My wife was able to drive that around perfectly on the first day without killing it, while she never could get the hang of the WRX. The Miata requires basically no clutch modulation, it's just all the way in, or not at all. The gearbox is extremely easy to move, yet notchy, so my wife didn't have to muscle the car into first and second like she did on my WRX.
The Gen1 Miatas are by far and away the easiest manual I've ever driven, and I recommend starting on one if at all possible.