You need to be looking at Act I scene 7 to find the answer to this question. Lady Macbeth reveals her plans to Macbeth after basically bullying him into committing the murder, pushing aside his doubts, worries and moral scruples:
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,
That memory, the warder of the brain,
Shall be a fume, and the receipt of reason
A limbeck only: when in swinish sleep
Their drenched natures lie, as in a death,
What cannot you and I perform upon
Th'unguarded Duncan? what not put upon
His spongy officers, who shall bear the guilt
Of our great quell?
Remember that Duncan is a guest in the castle of the Macbeths, so he is there as Lady Macbeth speaks. So, as it says, her plan is to wait until Duncan is sound asleep (as he would be exhausted after his journey). Then, Lady Macbeth is going to give the two henchman who look after Duncan so much wine that they will be drugged insensible, and then Macbeth can kill Duncan. Afterwards, they can frame the two henchman for the murder, allowing Lady Macbeth and Macbeth to get away with it.