Lady Macbeth says she'll wait until Duncan goes to bed (he'll be tired) and she'll make sure to give the two guards enough wine to make them sleep heavily. That way Macbeth will be able to sneak past them to kill Duncan with the two guards' daggers. Then they'll place them with the sleeping guards.
"When Duncan is asleep--
Whereto the rather shall his day's hard journey
Soundly invite him--his two chamberlains
Will I with wine and wassail so convince
What cannot you and I perform upon
The unguarded Duncan? what not put upon
His spongy officers, who shall bear the guilt
Of our great quell?"
The plan was to wait till Duncan went to bed, get the guards drunk and passed out and then Lady Macbeth would go and kill Duncan by using the dagger, yet he reminded her of her father so she couldnt and Macbeth ended up killing Duncan and the guards
He and Lady Macbeth plan to get Duncan’s two chamberlains drunk so they will black out; the next morning they will blame the murder on the chamberlains, who will be defenseless, as they will remember nothing. While Duncan is asleep, Macbeth stabs him, despite his doubts and a number of supernatural portents, including a vision of a bloody dagger. When Duncan’s death is discovered the next morning, Macbeth kills the chamberlains—ostensibly out of rage at their crime—and easily assumes the kingship. Duncan’s sons Malcolm and Donalbain flee to England and Ireland, respectively, fearing that whoever killed Duncan desires their demise as well.