Dear, Klairs Vitamin C

I have been wanting to try the dear, klairs vitamin c serum and I’ve had it stored in my fridge for the past couple months because I’ve never had the time to introduce it into my routine. Now that I’m at a time where I can, what is the best way to use this serum? Day or night? After or before which step? How many times a week? I’ve never used vitamin C before but I’ve heard this is a good product to try out first. For reference, I have my routines on my profile page. In the morning I use water to wash my face, the dear klairs hydrating water, I’m from mugwort essence, then my cerave sunscreen. At night I use the gentle black fresh cleansing oil, cerave foaming cleanser, dear klairs hydrating water, mugwort essence, innisfree green tea serum (sometimes), then any moisturizer I have on hand