Lady Macbeth reveals something of the plan to us when she tells Macbeth that he shouldn't have carried the daggers from the murder-scene:
Why did you bring these daggers from the place?
They must lie there. Go carry them, and smear
The sleepy grooms with blood.
In the next, scene, Lennox describes the scene as Lady Macbeth has left it - of course, it's her, not Macbeth, who returns to smear the grooms with blood:
Those of his chamber, as it seem'd, had done't:
Their hands and faces were all badged with blood;
So were their daggers, which unwiped we found
Upon their pillows:
They stared, and were distracted; no man's life
Was to be trusted with them.
The grooms "stare" and are "distracted" (can either be "distraught" or "maddened") because - of course - they haven't done anything. And then Macbeth admits - to everyone's surprise, including Lady M's - that he has killed both of the grooms.
So you see the ideal plan: but of course, in practice, it is botched twice: Macbeth first forgets to leave the daggers, and then kills the grooms on a whim (which makes him - rightly - seem extremely suspicious!).
The plan was for Macbeth to stab Duncan and then smear the sleeping guards with blood from the knives, leaving the impression that they had killed their master in a drunken fit during the night.