Hello Kirsten
The correct way to do it is by applying your foundation first and then your powder. The main purpose of your powder is to mattify a shiny complexion after foundation is applied. It seals it in and then prevents it from running in places like the creases around your mouth, your eyes if you’re applying it there and also to allow you smooth on other products like cream blush, cream eye shadows or ordinary powdered products without it looking shiny or over done.