a) Drill 3 small holes on the sheet, as near to the edge as possible, as far away from one another as possible.

b) Hang a plumb line from one of the holes.

c) When the sheet is stationary, trace the vertical line made by the plumb line using a marker.

d) Hang the plumb line from each of the other two holes and repeat the tracing of the lines.

e) The intersection point of the 3 traced lines is the centre of gravity of the plastic sheet.