First you'll want to lower the pool pH level. I found that 1 gallon of acid reduces the Y's pool by 0.2ppm. You may want to use 1 1/2 gallons, but there's a way you need to do it so it won't affect the alkalinity too much.
You can go to this link and find exactly how I do it.
The short of it is to put the acid around the perimeter of the pool starting at the deep-end and the filtration running. If you have any water features you need to turn them on to help with aeration. Keep the water moving when decreasing a pool's pH level.
Wait for one complete turn-over of the water. Usually state law requires a complete turn-over every 6 - 8 hours with about 240 - 280 gallons per minute (GPM).
Once the pH level is around 7.2ppm it's time to raise the total alkalinity. I have always used sodium bicarbonate and it works very well.