[Product question] First retinal eye serum

I just bought the Beauty of Joseon Eye Serum with Ginseng and Retinal. It’s my first time ever using a product with retinol/retinal, and I wonder how I should go about patch testing it? (don’t want to test it directly on my face, but idk where else?).

Also, is there anything I should keep in mind when starting with retinal based products? From what I’ve gathered the concentration of the serum is pretty mild, but I do have really sensitive skin. Any input on the product is greatly appreciated:)