Different ways to go about it. Preferably you would have it included in a mould. Secondary would be to form a negative of it and apply it to the helmet while still pliable to form the dent in place. Or you could build this area up with enough thickness to allow you to layout and physically grind out the needed material.
Maybe try one of the poster board helmet builds? You can always glass it when complete and you'd build it with the dent as you went. Several good paper build threads on here....