Ask them for their preferences, stuff they tried, stuff they didn't like, and if they are willing to try a new product, or stick with something that works. Based off of the customer's preferences, give options they can try out (this is where makeup and skincare knowledge can really help you out here)
Ask them for their preferences, brands they tried, stuff they don't want in the foundation. Provide different options for them, describe every product with pros and cons. Ask if they were looking for other stuff to go with it, such as: powder, setting spray, brushes...etc Try to be logical in your answer, and keep the customer in mind. Don't say anything to offend them.