I tried it two ways when i first received it. 

I had already introduced COSRX 's AHA/BHA clarifying treatment toner and coupled this with it and HOLY GRAIL for me (YMMV).

I did it for three weeks and saw an awesome reduction in acne scars that had lingered for months. I was expecting a slight tingle or discomfort to occur and it never did with the toner used before application.

I had also tried using it after cleansing (20mins for ph levels to re-adjust and I also have oily skin so I did not dry out) with no products before it to see if it would be more potent. I found that used in that manner I had a slight tingling but not uncomfortable. Decided the results were greater and more comfortable with toner and discontinued the second method after a week. Honestly this turned into must have and Ive already used a bottle up between me and my mother. Ive ordered two more, so she doesn't steal mine anymore!