At Michael's, I located a Martha Stewart glass etching cream (that came with a brush) and the corresponding stencils. The warnings on the back of the jar make the process seem like it will eat away at your skin (which it well may), so as I didn't have plastic gloves, I used plastic bags to protect my hands (and plastic under the whole set up too in order to protect my counters). I simply cleaned the glass, adhered the stencil, then generously applied the etching cream. I let the cream sit for 15 minutes, then washed it off and my letters stayed.
The project originated as jars for some Christmas gifts (for some reason the etching is almost impossible to see in these pictures...). These are hot chocolate jars (hot chocolate, marshmallows and crushed peppermints)
And the etching eventually turned to three of my pyrex dishes
For these, I thoroughly cleaned the pyrex dishes and flipped them upside down. I put the stencils on the pan backwards (which was an experience as they are sticky only for the correct way for the letter to face) and then followed the same steps from above.
It was really an easy way to personalize!