Here is a neat shot about how I installed a jalousie window. I placed the window on the wall and traced around it. Boards were glued to the wall on the outside of the tracing. I used a crude reciprocating saw to cut out inside the boards. The window fit snuggly into the opening. The boards give strength and plenty of purchase for the screws. I like using rope caulk, Mortite, to seal the window. It seals great, allows the window to be removed later and doesn't stop paint like silicone.