Dilute both oils to a maximum of 10 percent strength of that called for in a skin treatment recipe. Neither oil should comprise more than 10 percent of the skin treatment.
Start out with about 1/4 cup of each oil, and blend them together in an airtight container. Experiment with each oil amount.
Add less than the 1/4 cup of rose hip oil if the skin treatment will be used on acne-prone skin, or skin that has an allergic reaction to Retin-A.
Add slightly more avocado oil to the mixture if you will be out in the sun. While avocado oil is not a sunscreen, it will give you some protection from the sun's rays.
Add any essential oils that the skin treatment recipe calls for, and blend the oils together by swirling them gently in the airtight container. Refrigerate the mixture for up to a year when it is not in use.