Cut out a doorway from the side of the chicken coop you want the door to be placed.
Attach the wall siding you have just cut out to two perpendicular bracers on the inside of the door to stabilize it.
Screw two hinges on one side of the door, opening outward. They can be functional or decorative, but make sure they are element-resistant and won't rust.
Screw one or more doorstop lengths of lumber on the inside doorway of the coop so they overlap the doorway by 1/4 to 1/2 inch. This will seal the coop door when it's closed.
Adjust the door so that it is straight and flush with the doorway. Screw the door through the hinges to the chicken coop. Attach a bolt lock or other latching device to the door and coop to make the door secure.