She gets the idea in Act 1, Scene 5. What happens here is that, having just learned of the wonderful predictions for her husband's future (new titles, change in status, etc), Lady Macbeth wants to make sure that her husband gets what he has been promised.
Therefore, she says to herself:
"Glamis thou art, and Cawdor; and shalt be
What thou art promised: yet do I fear thy nature;
It is too full o' the milk of human kindness
To catch the nearest way: thou wouldst be great;
Art not without ambition, but without
The illness should attend it: what thou wouldst highly,
That wouldst thou holily; wouldst not play false,
And yet wouldst wrongly win: thou'ldst have, great Glamis,
That which cries 'Thus thou must do, if thou have it;
And that which rather thou dost fear to do
Than wishest should be undone.' Hie thee hither,
That I may pour my spirits in thine ear;
And chastise with the valour of my tongue
All that impedes thee from the golden round,
Which fate and metaphysical aid doth seem
To have thee crown'd withal."
In other words, she thinks to herself:
"Hmmm...too many things could go wrong here. My husband might not get what he has been promised if he is not forceful enough, or if he wavers between decisions, or if he is fearful of making a mistake, or if he is overly concerned about following rules and wanting to do 'the right thing'. Therefore, I'd better talk hard sense into him, to make sure he doesn't weaken. I'll keep reminding him that he should let nothing stand in his way."