How to Use Tretinoin and Reduce Irritation.

Don’t go all out with this stuff! Less is more here. Using too much could lead to inflammation, cause redness, and make hyperpigmentation worse!
Make sure that your skin is absolutely dry before applying it. Ideally, you should wait 20 minutes after washing your face to use it. Damp skin increases its topical permeability, which will make it more irritating.
Use it before or after your moisturizer. Applying it over moisturizer doesn’t reduce its efficacy and makes it less irritating. You only need to apply a pea-sized amount for the whole face. Yes, that is enough! It may not seem like it, but trust me: IT IS!
To reduce the side effects, simply begin incorporating it into your skincare routine more slowly. You can start using it once or twice a week and work your way up. For example, on week one use it Monday and Thursday. Week two on Monday, Thursday, and Saturday. Week three Monday, Wednesday, Friday, Sunday. So on and so forth.
Using products containing Niacinamide (e.g. CeraVe PM or EltaMD AM Therapy) have been shown to minimize its irritating effects. 
Because tretinoin will increase your sensitivity to the sun, it’s important that you wear sunscreen! And last but not least, make sure to apply it at night! Tretinoin degrades in sunlight.
To summarize: start slowly, use a pea-sized amount at night on dry skin before or after moisturizing, and wear sunscreen during the day. Use products with niacinamide to counteract irritation.
UPDATE 10/21/19: one study found that using short contact therapy is still effective and far less irritating versus leave-on treatment. This is what I’ve been doing very successfully with 0.1% tretinoin gel the past month. Do do this, simply apply tretinoin for 30 minutes on dry skin before thoroughly rinsing with water. You can buy tretinoin cream amazon
With this method, only 20% of participants in the above study experienced irritation, whereas it’s normally 80%! And if that wasn’t cool enough already, none of them were using moisturizer either!
Readmore Tretinoin Concentrations and Side Effects