How does Rosalind/Ganymede propose to talk to Orlando? Why does she say he can't be the one putting up the poems?

Orlando expresses his love for Rosalind by writing what can only be described (in a charitable sense) as love poems, which he then puts up all over the Forest of Arden in the hope that his beloved might read them. Less charitably, however, one would describe them as worthless doggerel, full of longing and emotion but with little or nothing in the way of inspiration.
What Orlando clearly needs is an education in the ways of romance. Fortunately for him, Rosalind (in the guise of Ganymede) is on hand to show him how it's done. She's flattered by the emotions expressed in the poems, even though she must know that they're pretty bad. In any case, she wants to meet this love-sick young poetaster and—still in the guise of Ganymede, remember—attempt to cure him of his crazy love.
She aims to do this by trying to get Orlando to imagine that Ganymede is actually his fair mistress. Then, he should try wooing her/him directly, and when he does, Ganymede will act like a spoiled child in the hope that Orlando will be put off love forever.
Before she does any of this, Rosalind playfully expresses doubt that Orlando really is in love. He certainly doesn't look it. For one thing, his appearance isn't in the least bit disheveled; he's impeccably dressed. Not only that, but he doesn't look like he's been experiencing sleepless nights. There's also no sign of misery or carelessness. According to Rosalind's uncle, these are all the symptoms of someone who's head over heels in love.