Act 1, Scene 5.
[To JULIET] If I profane with my unworthiest hand
This holy shrine, the gentle fine is this:
My lips, two blushing pilgrims, ready stand
To smooth that rough touch with a tender kiss.
Basically, he takes her by the hand and tells her that since his hands are so rough, he will pay for his indiscretion by kissing her hand.
Good pilgrim, you do wrong your hand too much,
Which mannerly devotion shows in this;
For saints have hands that pilgrims' hands do touch,
And palm to palm is holy palmers' kiss.
Since they are touching hands, Juliet suggests that palms against palms (or hands in prayer) is like a kiss done by saints.
Have not saints lips, and holy palmers too?
Seeing as they are touching hands, which Juliet suggested is like a kiss, Romeo asks why don't saints kiss with their lips instead.
Ay, pilgrim, lips that they must use in prayer.
Because, says Juliet, they are using their lips for praying.
O, then, dear saint, let lips do what hands do;
They pray, grant thou, lest faith turn to despair.
Romeo says that their lips should do what their hands are doing -- touch each other in prayer.
Saints do not move, though grant for prayers' sake.
Juliet, to shy to make the move, says that saints don't move.
Then move not, while my prayer's effect I take.
Romeo assures her that she doesn't need to move, he will move, and kisses her.
Thus from my lips, by yours, my sin is purged.
After the kiss he explains that his sins have now been taken away by her lips.
Then have my lips the sin that they have took.
Juliet laments that now Romeo's sins are on her lips.
Sin from thy lips? O trespass sweetly urged!
Give me my sin again.
So Romeo states that he will take his sins back, and kisses her a second time.
You kiss by the book.
You can read a modern English translation of the text here, and watch a nice modern adaptation of this kiss scene here.