Examine the space between the top of the door and the door frame. You are likely to notice more space between the door and the frame further away from the hinges.
This is a sign that the hinge screws are being pulled loose.
Fold up some newspaper or a magazine and place it under the door. The magazine or newspapers should be thick enough to keep the door level and prevent it from falling to the floor once you remove the hinge pins.
Use a flat-head screwdriver and a hammer to remove the hinge pins. Place the edge of the screwdriver against the edge of the pin.
Tap the end of the screwdriver with the hammer so that the pin is lifted up and out of the hinge. Repeat for each door hinge, then set the door aside.
Unscrew each hinge piece from the door frame with a screwdriver or a power drill.
Insert a toothpick into each of the hinge screw holes. Snap the toothpick off or cut it so that it is flush with the surface of the frame.
The toothpick helps keep the new hinge screw steady in case the old screw widened the hole as it was pulled loose.
Reinstall each hinge section to the door frame with 2-inch-long wood screws. Each hinge section typically requires three screws.
These screws bore deep enough into the frame that they reach the support stud. This provides extra stability and prevents the door from sagging or becoming skewed again.
Set the rolled-up magazine or newspaper on the floor in front of the door frame. Place the door on top of it, then align the hinges together.
Insert each hinge pin back into each hinge, and tap it back down into place with your hammer. Remove the magazine or newspaper from beneath the door.