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Use a well finished medium-density fiberboard for the doors. The size of the pieces will depend on the chosen height of your chicken coop. Each door should be as tall and half as wide as the door opening. Screw a 2x2 along each side of each door opening, as well as along the tops. This will give you a sturdy place to screw the door hinges. Screw in two hinges per door—one about four inches from the top of the door and the other about four inches from the bottom. Note that you may need a third hinge directly in the middle, depending on how tall your chicken coop is. You can use the same measurements for the back of the coop as you did for the front, but remember to take new measurements for the doors on the side of the coop. Brass hook catches are an inexpensive, efficient closure to use, but any type of closure will work, so long as it is not easily opened by common predators such as dogs or skunks.

Summary:
Cut the wood. Install a door frame. Attach the front doors. Repeat this process for the other two openings. Add closures.