First off you'll need that special paint. And a b&w scan of your picture. Print it out on a label, and then with a Xacto knife cut the lines. Peel it off and place on your glass or whatever, and then using a paint brush, paint that glass etching paint on. Let it dry for a good while and then peel off the sticker, so the parts without the sticker will be the ones to have been painted, thus giving you the etched lines. Here's an example:
You would score on the black lines and then peel off the places where the grey part is because that's what you want etched. Place the sticker on the glass and then paint it over top of it.
If you need some help let me know, I did one on my bathroom mirror. Just a plain design, no Futurama!