What advice does Lady Macbeth give Macbeth when he arrives home?

When Macbeth arrives at their home, Lady Macbeth tells him, "To beguile the time, / Look like the time.  Bear welcome in your eye, / Your hand, your tongue" (1.5.74-76).  Macbeth looks too guilty, so she tells him that in order to deceive everyone, he's going to have to look more normal.  He must graciously and hospitably welcome the king and his retinue to the Macbeths' home in every way that he would typically do so: in his looks, in what he offers them, in how he speaks to them.  

Further, she says, "Look like th' innocent / flower, / But be the serpent under 't" (1.5.76-77).  In other words, then, Macbeth needs to appear to be as harmless and simple as a flower, a symbol of good feeling, but actually act as the concealed snake underneath.  Because of the story of Adam and Eve in the Garden of Eden, snakes are often symbolic of evil (because the snake in the garden was really the Devil in disguise).  Therefore, Lady Macbeth wants Macbeth to deceive the king and everyone else so that their suspicion does not alight on the couple.