Cornstarch for butt rash

em

Our Ped said to use cornstarch for his butt rash and I realized I just said “Ok” 🤦🏽‍♀️🤦🏽‍♀️🤦🏽‍♀️ like I have no idea what I’m doing or what it even is 😩 what brand? Where do I get it? How do I apply it? Do I mix it with something?? Please help this mama if you can