What does Lady Macbeth do to plan Duncan's death?
In Shakespeare's Macbeth, Lady Macbeth raises the idea of assassinating Duncan to her husband, plans the assassination, talks her husband into the assassination, and drugs the grooms who are assigned to attend to Duncan during the night.
When Macbeth tells her that Duncan is spending the night in their castle and plans to leave in the morning, she, in effect, says that the sun will never see the day during which Duncan will leave their castle, thereby suggesting that they kill him. She tells Macbeth to leave it all to her, and she plans to drug the grooms so that she and her husband can kill Duncan. She does so.
When Macbeth decides not to go ahead with the assassination, she berates him and manipulates him into going ahead with it.
She drugs the grooms and Macbeth kills Duncan as planned. You can find the above in Act 1.5 through Act 2.2.