In order to obtain the rhyme scheme, you can analyze the poem as follows, each letter is assigned to the sound in the end of the line, changing the letter every time the sound changes.
My mistress' eyes are nothing like the sun; A
Coral is far more red than her lips' red; B
If snow be white, why then her breasts are dun; A
If hairs be wires, black wires grow on her head. B
I have seen roses damasked, red and white, C
But no such roses see I in her cheeks; D
And in some perfumes is there more delight C
Than in the breath that from my mistress reeks. D
I love to hear her speak, yet well I know E
That music hath a far more pleasing sound; F
I grant I never saw a goddess go; E
My mistress when she walks treads on the ground. F
And yet, by heaven, I think my love as rare G
As any she belied with false compare. G
ABABCDCDEFEFGG
