Inspect the roof and attic areas of the exterior of your house. Bats commonly enter through attic vents or gables. The screening that normally covers attic vents can wear down over time, creating holes that bats can enter through. Cover each possible entry point with bat screening. This flexible screen material is UV protected and will not be worn down. It is pliable and can be stapled down to cover openings. Leave a couple of the entry points open.
Wait until night time, when bats leave the roost to search for food. Because bats are less active in the winter, it may not be possible to be sure that all the bats have left. One solution for this is to cover all of the openings but leave the bottom of the screen unstapled. Bats will be able to escape by pushing their way out the bottom, but they will not be able to re-enter. There are also commercial bat control devices that can be installed and work the same way. Not only is this more humane, but it will prevent bats from dying inside the attic and decomposing there.
Wait for several weeks until you are sure that all bats have evacuated. In the summer this only takes a few days because bats are more active. Then remove the bat control devices and cover the gaps with the screening material. If you left gaps in the screens, staple the bottoms so that they are completely secure. Check all of the other openings to make sure that the screen is still secure.
Clean up after the bats. Unfortunately, the damage left by bats does not end when the bats leave. They leave piles of droppings and urine that breed bacteria that is harmful to humans. The mess left by bats, called guano, can grow a fungus that breeds spores. It can become airborne and inhaled. The urine can also saturate and warp wood and damage insulation. It is recommended that you hire professionals to clean this up, because of the danger involved.