Lady Macbeth does a couple of things in this scene.
Lady Macbeth: That which hath made them drunk hath made me bold:
What hath quench'd them hath given me fire.--Hark!--Peace!
It was the owl that shriek'd, the fatal bellman,
Which gives the stern'st good night. He is about it:
The doors are open; and the surfeited grooms
Do mock their charge with snores: I have drugg'd their possets
That death and nature do contend about them,
Whether they live or die. (II.ii)
The lines above show that the "fair and foul" theme applies to when she drugged the drinks of the guards who are to watch over King Duncan. For her, their drugged drinks were fair because they gave her courage--she would have murdered Duncan herself but he looked like her father, "Had he not resembled / My father as he slept, I had done't"--yet for them the drugged drinks were foul because drinking them sabotaged the protection of Duncan. Later in the scene, she helps to frame these guards for Duncan's murder. Macbeth has killed the king, but still has the bloody daggers. She says the following:
Lady Macbeth: Infirm of purpose! Give me the daggers: the sleeping and the dead Are but as pictures: 'tis the eye of childhood That fears a painted devil. If he do bleed, I'll gild the faces of the grooms withal; For it must seem their guilt. (II.ii)
Basically she is telling her husband that he is foolish to be afraid of the dead body--to think of it as a picture, as something not real. She then takes the daggers and smears the king's blood on the sleeping guards to make them look like the murderers and leaves the daggers by them. Ironically, it is she who later has difficulty dealing with the blood on her hands.