What did Maria plan in Twelfth Night?

In Shakespeare's Twelfth Night, Maria is Olivia's waiting-gentlewoman and a central figure in Olivia's household.
Maria intends to humiliate Olivia's dour, pompous, prideful, self-righteous steward Malvolio for his constant criticism of her and other members of Olivia's household, including Sir Toby Belch; his friend and companion, Sir Andrew Aguecheek; and Feste, the fool.
Her plan is simple. She'll write a letter in Olivia's handwriting that will lead Malvolio to believe that Olivia is in love with him and leave the letter where Malvolio can find it.

MARIA: I will drop in his way some obscure epistles of love;wherein, by the colour of his beard, the shape of his leg,the manner of his gait, the expressure of his eye, forehead,and complexion, he shall find himself most feelingly personated.I can write very like my lady, your niece; on a forgot-ten matter we can hardly make distinction of our hands. [2.3.143-148]

Sir Toby approves the plan:

SIR TOBY: He shall think, by the letters that thou wilt drop, thatthey come from my niece [Olivia], and that she is in love with him. [2.3.151-152]

Malvolio does, indeed, fall for the trick, and he believes that Olivia is in love with him.
Maria writes in the letter,

"Remember who commendedthy yellow stockings, and wished to see thee ever cross-gartered:..." [2.5.137-139

And,

"If thouentertainest my love, let it appear in thy smiling; thy smilesbecome thee well; therefore, in my presence still smile, dearmy sweet, I prithee..." [2.5.155-158]


MALVOLIO: Jove, I thank thee. I will smile; I will do everything thatthou wilt have me. [2.5.158-160]

And he does. He appears to Olivia wearing yellow stockings, cross-gartered, and he smiles incessantly when he's in her presence.
Of course, Olivia thinks he's gone mad—

OLIVIA: Why, this is very midsummer madness. [3.4.53]

—and Malvolio is well and truly humiliated.

OLIVIA:Alas, poor fool, how have they baffled thee! [5.1.382]