Hi, Kristy. Before you start cutting holes, it might be nice to be sure that you are not trying to put the hole where the ceiling joist runs. This is a big thick piece of lumber that runs from the outside wall of your house toward the other outside wall. Since you know one is there, you can either take an electric drill and drill small holes every inch to see what you hit. If you hit nothing in the area where you want to install the fan, you will be OK. If you hit wood, you will have to change the location slightly. Another way is to drill a hole on the center of the square and push a wire up through it then go up in the attic and see where it comes out. Those pieces of wood you are walking on are either the joists or flooring laid on the joists. When you find where the wire is, lay out the size of square you need up there between the joists and drill the 2 corner holes, like John said. Then you can go back downstairs and use the corner holes to locate the square there. Then you cut the square hole with the saber saw.