I am helping a friend to renovate an old school building that was built in 1922. Right now the windows that are in what is going to be the living room are single pain aluminum windows that are attached by a little bit of mortar. They are attached right to the masonry. The previous owner had bought new vinyl windows to put in but never got around to do it. I am looking to find out the best way to install the new windows to the existing rough openings. Should I build new window frames to the rough opening? Also, should I install flashing to help with the weatherproofing? I've been around construction most of my life but have never done a project like this before. He is a 40 hour a week guy and I am laid off from the concrete construction industry right now so we are trying to do this project on our own. I am going to try and attach some pictures to give a better idea of what I am talking about. Thanks in advance for any help you guys could give me.