Measure from the right hand side of the grid to the edge of the light fixture or ceiling diffuser, using a tape measure. Transfer this measurement to the ceiling tile, which will be placed in the grid square.
Measure from one of the grid sides running perpendicular with the right side to the edge of the light fixture or ceiling diffuser. Transfer this measurement to the same ceiling tile.
Measure the diameter of the ceiling diffuser or light fixture. Open a compass to this diameter. Place the pencil end of the compass on one of the two lines and the pointed end of the compass on the ceiling tile in what will be the center of the opening for the diffuser or light fixture.
Trace from one mark to the next mark. The pencil must hit each mark as it goes by. Continue arcing the compass until you have traced a completed circle.
Place the cutting blade of a utility knife onto the traced circle. Apply gentle pressure and cut around the entire circle. Do not bend or twist the circle to remove it. Continue cutting through the ceiling tile until the circle easily slides out.