Climb a ladder so you have easier access to your ceiling. This allows you to find the ceiling joists without straining.
Run a stud finder over the ceiling to locate the ceiling joists. The joists provide support for the screws needed to hold the mirror in place so it doesn't fall.
Ask an assistant to hold the mirror up against the ceiling so its pressboard frame is against the joists.
Drill long screws through the pressboard into the ceiling joists every 5 or 6 inches around to ensure that the mirror is supported fully.
Apply an epoxy glue to the pressboard to attach a frame to the mirror if you wish to do so. Follow all instructions for your individual frame for best results.