Use a level to determine where the ceiling is sagging. Draw guide lines along the wall where you want the ceiling to hang. Use the level as a straight edge to make the guide lines.
Screw brackets into the joists, spacing the brackets 15 inches apart. Screw metal channels into the joists.
Measure the ceiling. Cut drywall to the dimensions of the ceiling. Have an assistant hold the sheet of drywall onto the ceiling while you screw it into the wall with drywall screws and a screw gun. Start screwing from the middle of the panel of drywall and place screws about every 8 inches. The screws should not be so tight that they tear the paper covering on the drywall.