One of the most important design elements in building any bird house is the hole size. The hole diameter will determine who or who cannot use your wren nest box.
For House Wrens, Winter Wrens, you can make the hole diameter 1 1/8″ which is also large enough for black-capped chickadees.
A 1 1/4″ hole will allow Carolina Wrens and White-breasted Nuthatches to use the box, but also will allow unwanted house sparrows to enter as well.
You can place the nest box 5-10 feet above the ground, under the eaves of a building or a tree limb. Wren boxes can also be firmly fixed to the side of a building or tree or can be hung freely from a wire. Carolina wrens prefer a house that is more hidden, like around shrubs or trees, than exposed and in a wide open space. We have seen wrens build nests in hanging plant baskets and even in the mouth of an old flounder skeleton that was mounted on the outside wall of a shed!
To attract wrens to your backyard and to ultimately use your bird house, place suet, peanut butter or meal worms in nearby feeders.