Cut the opening 2" wider on each side than the door frame, and install a 2x4 between the wall and the door on each side. Use screws and (eeeek! I am having a brain fart, the plastic inserts you put into concrete so you can insert a screw into them are called...)
Yeah, so that's how you do it. Then you can screw the door into place as needed. Don't forget the foam.