When a door is installed properly there is only a fraction of an inch between it and the frame. This helps to make the tightest seal possible while the door swings freely on its hinges. On occasion, however, the house may shift or settle and the space between the door and the frame may change so that the door will not open and shut properly. In this case you may have to remove some of the door's height so that it will function properly. If you are unable to take the door down for some reason, you can still do this with the door on its hinges.
Lay the tarp out so that the door will open over it. Open the door and make sure it moves freely over the tarp.
Shut the door until it touches the frame. Draw a line across the top of the door where it touches the frame.
Place the chair against the wall so that it will be between the door and the wall when you open the door. Open the door so that it is touching the chair and place the ladder against the other side of the door; this will brace the door while giving you some space to work.
Climb the ladder until your chest is to the top of the door. Plane the top of the door down until you can no longer see the line you drew. Make sure the door shuts properly, and if it does not, plane it down until it does.
Sand the top of the door to a smooth even finish.