Measure the length of the room -- from wall to wall -- with a tape measure to determine the runner length required for the suspended ceiling.
Pull the tape measure along a length of ceiling runner. Mark the required cut length on the painted front face of the runner with a pencil.
Set the base of a combination square against one side of the ceiling runner. Align the blade of the square with the pencil mark. Pull the pencil along the blade of the square to mark a perpendicular cut line.
Cut the marked line up to the center support tee with a pair of aviation snips. Move to the other side of the support tee and cut along the pencil line to the support tee.
Turn the ceiling runner over so the cut painted side is sitting on a flat surface. Place the aviation snips over the top edge of the runner. Cut toward the cuts on the front face of the runner.
Bend the runner gently back and forth at the cut to break the cut free.