Have a helper hold the cabinet against the wall, positioned at a comfortable height (see Overview for guidelines). Check it for level and plumb. When you're satisfied with its position, trace the cabinet's outline onto the wall in pencil.
Use a keyhole saw to cut along the pencil line. Remove the drywall inside your cut line, being careful not to mar the remaining wall. You may have to break off pieces of drywall still attached to the studs.