I haven't replaced a BISF door frame myself but one of our former members 'Rusla' undertook and photographed a similar project to replace the rear timber door of their BISF house, replacing it with a UPVC unit some years ago.
The original gallery of the project was lost during a site update but I have managed to locate a back up of their fascinating project. The images at present are in no particular order and sadly contain no Exif data which would allow me place them in the correct sequence.
I'll try and sort them visually myself and upload them for you shortly.
Although the project relates to a rear kitchen door, you should find the framework and original fitting method to be the same.