I love to etch glass, it is one of those completely rewarding projects with a big reveal at the end. It's pretty simple, you make a vinyl stencil for your surface and apply etching cream to the area. There's a couple things to remember, so keep reading...
I found the seashell shapes I liked in the Silhouette store, and sized them appropriately for the glasses. Then I "offset" the design so there was a 1/2" border (more or less) around the design. Don't forget to weed the positive space (the part that should be etched).
In this next photo you can see the little station I set up to work. A bag of rice held the glass steady. I wiped the glass clean with an alcohol pad before applying the stencil. I had my design ready to go.
So I applied the vinyl to the glass (with transfer tape) and painted on the etching cream.
Then I let the glasses dry as flat as possible for a few hours (NOT just a few minutes like the bottle indicates). These were weighted on the bottom so that they tilted nicely. You wouldn't want them to be upright and drip.
After a few hours I rinsed the extra cream off, took off the stencil pieces, and scrubbed with soap and a potato scrubber to get all the residue off. It's a funny feeling scrubbing away at the design, like you're going to scrub it right off. But that's the beauty of etching, you have irrevocably damaged the glass and you can't undo it.
Here is the etching cream I use with my affiliate link if you're inclined to use it :)
And of course, my beloved Silhouette Cameo (affiliate link):