The first step is to load your mirror onto the laser machine's bed. You can use the aluminium table or the honeycomb table for this tutorial. Make sure the mirror is the right way prior to processing.
After that you can engrave the mirror reflective side up side through the glass with your desired pattern. Glass does not react to the fiber laser, so only the mirror layer is removed when laser engraving. The glass itself is not scratched because it reacts only to the wavelength of the CO₂ laser source.