I did some years ago and was pretty easy and fun. I would love to get my hand on some more etching cream and do some more.
I traced a design onto a tinted contact then put that onto a cheap mirror and carefully cut out the design then put the etching cream on it. I ended up with a fabulous mirror with a leopard etched into it. (I would love to know what happened to that mirror lol)
Be careful as to which craft knife you use, some will scratch the glass if you press too hard. A glazing company or window tinter should be able to advise you as to which ones are ok.