How do you apply lemon juice to your acne scars, straight from a lemon and for how long do you keep it on?

Neither. Lemon juice will not get rid of acne scars. If you are using it as a bleaching agent do not use it straight. If you are going to try it use it diluted in a moisturizing cream base to a small area to test the effect.
Not familiar . While many fruit acids such as citric acid may help exfoliate the skin, I am not familiar with the use if raw lemon juice on the skin and it is likely to cause some irritation.