Banquo congratulates his best friend Macbeth for his victory. They are on their way back home.
Three witches stop them and tell then their future. Macbeth shall be thane of Cawdor and also King. Banquo will be greater but lesser then Macbeth.
So I just go in there and stab him?
Yes. It's very simple unless you are not man enough.
Macbeth comes home and tells his beloved wife that he might become King soon and what the witches have said has come true. She doesn't believe at first but then soon hatches a plan to ensure their royalty.
Hahaha! The deed has been done!
King Duncan is fast asleep. His two guards are knocked out cold by Lady Macbeth. She drugged their drinks in hope of killing them so they could murder the king without any witnesses.
Macbeth and Lady Macbeth plot the murder and plan to smear blood on the guards to make it look like they are responsible for the murder.
Lady Macbeth and Macbeth have now murdered King Duncan to ensure Macbeth becoming King for sure. Even though the murder is done and they wash their hands, they will never be clean again.