Measure from one edge of the grid square to the approximate center of the can light, using a tape measure. Transfer this measurement to the tile and mark with a pencil.
Make a perfect circle the size of the opening of the can light, using a compass. The mark you just made will be the center of the light.
Cut out the circle with a utility knife. It does not have to be perfect. Can lights come with covers to help hide imperfections.
Lift the tile into the grid square and gently lower it into place. The can light will line up with the circle you cut in the tile.