clean your mirror. Make sure the back is clean and nothing is sticking to it anymore.
Make your design file ready or download mine. Make sure you reflect horizontally your design. Because you gonna laser it on the back of the mirror.
Laser your design on the back of the mirror. Do a framing before you process it, then you will be sure it will fit.
You can add a led on the back. It give a nice extra touch, but it's not necessary.