My mother found two bat houses in a garage sale years ago.
She cleaned them out and had one of my brothers raise a pole above our shed and nailed the house ontop of the pole - She said bats aren't too fond on living in a home nailed to a tree. (?)
The houses were not large, very narrow ... They pointed to the sun (keep the babies warm) ... 6 feet above the shed
Each year my mom has about 5-10 bats living in these two little homes.
It's amazing to watch them as we have bonfires and as the bugs fly towards the light, we'll see some swoop...
The most important thing to her is that they do their jobs at night while the birds are asleep - during the day they allow the birds to munch away.
I found some instructions - It's a little to technical for me ....... (again, I'm the one who thoroughly enjoyed making a milk carton bird feeder last week)