Step One: Find a new door. There are many varieties of door and you need to decide on one that will fit your budget and match the style of the rooms.
Step Two: Remove the door by pulling out the hinge pins to separate the hinges. Leave the hinge brackets on the door and the wall for now, even if you plan on replacing them. They will serve as a template for applying the new brackets.
Step Three: Use the old door as a template. Lay it on top of your new door to make sure that they are the same size. If not, there are two options. Either trim the top, bottom, and far edge of the new door to make it fit, or re-size the door frame. Trim with a circular saw if the discrepancy is large, but use sandpaper if it is small. Re-sizing the door frame is far more complicated and should be avoided if possible
Step Four: Trace the hinge locations of the old door onto the new door. Line the door edges up, new door on bottom, and then push the old door slightly back. Draw lines to mark the top and bottom of each hinge on the bottom door.
Step Five: Put hinges on the new door. Unscrew the hinges from the old door or buy new ones. If you buy new ones, you will need to also unscrew the hinge piece from the door frame and replace it.
Step Six: Rehang the new door to make sure it fits. Mark any places that may drag or not slide into the door frame. If it does not fit, take it down and repeat step three, using the areas you marked as a guide. It is important that the door can open and close easily.