Here comes the tricky part.
It is important that you know that this product should not replace cmc or tylose powder. That is, do not abuse cornstarch. You can use it on occasion but it is not the best option.
When you use it, try to use small pinches with the piece of fondant that you are going to make, but be careful not to go too far or you can make it dry too quickly and be very difficult to handle. How much quantity? It will depend on where you live: humidity, heat, temperature…