Measuring for & Installing your French Door Pet Door is very simple. (You must have individual -removable panes of glass. You cannot cut an opening into a single-solid pane of glass, your entire door glass will shatter as doors are made with Tempered-Safety glass.)
Step A: Measure the exposed glass pane as you see it; without removing the molding. The door made for you will have a flap that is 2" smaller in width & height. (Make sure this resulting flap size will be large enough for your pet.)
Step B: Remove the molding around the selected pane of glass where the pet door will be installed. (If you are using more than one pane, remove molding around area.).
Step C: Remove the pane of glass and put the molding back around the opening (without glass pane).
Step D: Your pet door comes apart into 2-frames. These frames sandwich the opening and adjust to the molding thickness. Drill pilot holes for screws (the 2-frames each have a lips that glide over each other to adjust to the molding thickness. Where these two lips overlap is where the pilot holes are drilled). Put screws into pilot holes and tighten.
Note: you may at a later time remove the pet door and re-install the pane of glass.