How do you combine this product? Meaning which other products or active ingredient do you use in your routine when adding this one?

When you start using this, you'll want to introduce it to your routine slowly at night and limit the number of actives you use at the same time. Retinol can increase sensitivity and you need a sunscreen in the day. Once you're used to it, you can play around with things a little to see what works for you and what you can tolerate. Everyone is different.