Drag a permanent marker across the top of the ceiling grate to mark the cut locations. Mark each slat of the grate in a circular pattern for round holes.
Open a pair of straight-cut aviation snips. Slide the snip jaws over a marked slat. Squeeze the snip handles together to cut the slat. Move along the grate, cutting each marked slat as described. Discard the cut piece of grate.
Set the grate on the floor or worktable with the cut section facing toward you.
Drag a utility knife blade over any leftover stub to smooth it down.