There are ready-made barn owl boxes for purchase, or you can make your own.
For do-it-yourselfers: nesting boxes can be made from plywood. There needs to be a minimum floor space of 12" x 12", and a minimum depth of around 16". If you're able to make it larger, a partition to separate the entrance from the nesting area is useful to help protect the owls and their babies from predators.
The roof should be slanted to allow water to run off. Drill holes in the floor near the corners to let water drain out. Also drill holes around the top near the roofline for air circulation.
The nesting box will need to be designed with easy cleaning in mind. Common ways to clean out the box are to have a floor that drops out (warning: this method means that once you release the floor for cleaning, everything in the box will drop out -- dead animals, castings, whatever is in there... and you won't know what it is until that moment); or have one side of the box that opens up for easy cleaning.
When the nest is unoccupied, cleaning can be beneficial for the owls to remove the accumulated debris in the nest or to clear out wasps or other insects that may have moved in.
Mount the nesting box 10 to 15 feet above the ground. This is high enough to attract the owl but low enough that the box can be more easily and safely cleaned. The box can be mounted on a pole or in a tree, or in an existing outdoor building like a shed. Place the box away from the house (and cars, etc -- owls are very messy) but not too far from the yard.
A barn owl is a wild animal and can transmit health risks to humans. The biggest risk is from hantavirus, primarily carried by deer mice, one of the rodents consumed by barn owls. Owl castings (the undigested hair and bones of their prey), uneaten or partially eaten rodents, the owl's feces and nesting material, could all carry the hantavirus.
Do not attempt to handle the owl yourself. If the owl appears to need help, call the local wildlife rehabilitation center for advice.
(And if you're not really into having a real barn owl on the property, then sometimes an owl garden statue works too - check out the ones that can turn their heads or light up their eyes!)
While a resident barn owl can be a great tool in the fight to learn how to get rid of mice naturally, it shouldn't be the only tool used. There are many things homeowners can do to minimize rodent populations in their yards and gardens, and thus help to prevent them from entering their homes too. See the article, Preventive Pest Control – Do-It-Yourself Tips for Homeowners for more information.