Show by direct reference to his speeches that malvolio is self decieved before he is descieved.

Take, for example, Maria's letter written in the style of Olivia. It isn't so much that Maria has a talent for writing forgeries more than  she knows exactly what Malvolio would want to hear. Malvolio dreams of marrying Olivia so he can increase his social standing along with the money that comes with it. After Malvolio reads the letter, he is so happy that he neglects to consider the authenticity of it,

"I thank the stars, I am happy. I will be strange, stout, in yellow stockings, and cross-garter'd, even with the swiftness of putting on. Jove and my stars be prais'd!"