Rule Two: The roof must not leak. If you have an owl box with a gable roof, make sure the joint is watertight, When in doubt, add flashing across the top of the roof line. I learned this the hard way. This earlier post has photos of my old gable-roofed nesting box. 

  Rule Three:  Once you have the box, put an inch or two of wood shavings on the floor.  I have used chips from a wood-chipper, and I did not get an owl until I switched to the wood shavings that you can buy in a pet shop. Maybe it w as a coincidence, but I had a male Screech Owl back in the box in days last winter after I switched.
   Owls normally nest in tree cavities, and the cavities normally just have bark debris and fine wood chips – the intent is to mimic this in your nest box. 
    Rule Three A: If you plan to install a mini-videocam and microphone in your box, do it before you put the box up! You do not want to try to install a camera -- let alone try to focus it -- once the box is mounted up a tree.  
   Rule Four: The box should be placed on a tree or pole at least 10 feet off the ground. This makes it harder for predators and curious humans to get to it. 
    Use a ladder to reach the nest box for checking and maintenance.