Second Half Method
The moment you have the 4 molding pieces cut, put them together and check for the accuracy on the mirror. Use sandpaper or a coping saw if the joints do not fit well. Most of the time the corners are irregular in its shape, therefore, make use of construction adhesives and apply at the bottom, sides, and top of the mirror. Also, pick one molding, and apply a drop of the adhesive on it. Place that piece on the wall mirror and press it till the adhesive gets excessively sticky. You can punch in some finishing nails too.