Clean glass with acetone or suitable cleaner.    I used acetone because I have it readily available as it is used in my business ( Fiberglass Manufacturing).

Spray Area to be engraved with “SPRAY CHALK”.   I purchased it at Michaels ($6 Cdn/can).  Works awesome and easy, easy, clean up.   Spray 2 - 3 light coats allowing to dry for a couple of minutes in between coats.  It goes on smoothly and drys quickly ( I allowed about 15 minutes after the second coat for this)   It comes in several colors (which I don't think it matters what color you use).  I used blue for this application

Nice Clean Deep Etching onto the glass.  

When done rinse glass with warm water.   Soo  So Easy!