Instruct the male to lay on his back. During foreplay, make sure the male becomes erect prior to assuming this position.
Advise the first female to lower herself on the male's penis as if she were "on top" during sex.
Ask the second female to lower herself over the male's head, so that her vagina is over the male's mouth and she is facing the other female.
Engage in sexual activity whereby the first woman is having sexual intercourse with the male, while the male is performing oral sex on the second female. Meanwhile, the two females, who are facing each other, fondle and play with each other from the waist up. As the two women caress, you will see the shape of the "Napoleon's hat."
TIP: When trying to invoke this sexual fantasy, be sure to maintain open and honest communication with your two other sexual partners.