http://teotwawkiblog.blogspot.com/2013/01/something-useful-from-doomsday-preppers.htmlQuick description: the window is opened, and a long bolt fed in through a hole in the pre-cut plywood and then through a 2x4 cross brace on the interior of the home. Tightened the bolt down and you're good to go. They're using 3/4 inch plywood and, in Doomsday Preppers style, spend some time trying to smash through it. Not bullet proof, but it will certainly slow someone from getting into a home and provide concealment, too.