Wood is a natural product and its water content varies according to prevailing environmental conditions, especially if the wood is unpainted. The temperature and humidity of a room will affect the water content of the door and the frame. Sometimes, the water content of a wood door causes the door to swell and stick in the frame.
Open the door, using force if necessary but taking care not to damage the door. Place wedges under the door to support the weight. Unscrew the hinges by the screws going into the frame. Move the door away from the frame.
Measure the distance accurately between the right and left faces of the frame inner. Measure in several places to check that the measurements are the same, as they should be.
Measure the distance accurately between the top face of the frame inner and the floor or threshold. Measure in several places to check the measurements are the same, as they should be. Check that the interior corners of the frame are right angles, as they should be, using a set square.
Fix any problems discovered with the door frame before continuing, to make it rectangular. Measure the inner frame dimensions again, using the same method as before.
Transfer these measurements to the door with a pencil and carpenter's rule, starting from the hinged edge. Plane the other door edges as required to make the door the same size as the interior of the door frame. Plane an additional 2 to 4 mm off the non-hinged side of the door, so that it will fit in the door frame and open and close easily.
Stand the door by the frame and place wedges under it to support the weight. Reattach the hinges to the frame. Test that the door opens and closes easily.