You can either pour it in the skimmer or in front of a return jet. The pool store may or may not have a system that has a ph scale that differentiates between 6.8 or lower. Many don't go lower than that. Hopefully, it is 6.8 and not lower.

The key to getting rid of algae is to sustain the high chlorine levels and not let it yo-yo up and down. It is best if you can test it at least twice a day and three times is even better and each time dose it back up with chlorine. Since you say that your cya level is around 20, you'll want to try and keep your chlorine at about 12ppm.

Glad to help. Keep us posted how it is going.