Stabilize the sections of undamaged ceiling around the opening with drywall screws placed a little back from the edge—this helps to avoid crumbling or buckling of your nice, clean-cut edge. Pic 1.

Use enough pressure when driving screws so that the screw heads imbed slightly into the wallboard surface.

Center rafter screws go in straight.

Hold the screw gun at a slight angle and drive screws into the side rafters, starting the screws a little back from the edge as before. Pic 2.

Screws in side rafters are angled because you are fastening to half a rafter—a straight-in screw may miss the rafter altogether.

If the opening needs insulation replaced, do it now.