Elements include Costuming, Sound, Lighting, Characterisation and Voice, Props, Blocking, Set design, etc.
Oooo! I would do more than just an "imaginary baby"! I would have her pick up some kind of doll, ... possibly one with a porcelain head and after holding it to her breast indicating that she has "given suck," ... smash the thing to bits! Add a sound effect there: "CRASH, ... cllink, clink, plink, plink" as the bits scatter on the floor. Or, even better, maybe there could be some kind of red goo in there that could seep out! (Cool!) The lighting? Blood red.
I think key to this is the role of Lady Macbeth, and so you will want to think through the characterisation of this important person for this scene. She is clearly dominant, and one way you could show this would be to have her moving closer towards Macbeth as she delivers her lines, and for him to move backwards away from her slightly. She would obviously deliver her lines in an angry voice, almost spitting with rage and with derision. Macbeth might be even shocked at the vehemence in her voice and what she says:
I have given suck, and know
How tender 'tis to love the babe that milks me:
I would, while it was smiling in my face,
Have pluck's my nipple from his boneless gums,
And dash'd the brains out, had I so sworn
As you have done to this.
Obviously, such lines would be delivered with appropriate gestures, such as throwning an imaginary baby on the floor when Lady Macbeth says "dash'd the brains out." Thus the intereaction between Lady Macbeth and Macbeth is incredibly important to suggest how she is manipulating him. I would also want to use lighting to create a lot of darkness, with perhaps the two characters being surrounded by darkness, and Macbeth moving into darkness as he decides to continue with the murder as originally planned at the end of the scene. This would suggest that Lady Macbeth has been successful in her manipulation.