As you can see, these are almost opposite ways of etching; in the first, the etched glass (the opaque portion) is the negative space, and in the second, the letter is etched glass is the letter. If you're looking for some tips doing your own etching, here's what I do: in Microsoft Word, I pick the font and letters I want to etch, put it in a text box, then flip the text box so it is reversed. I trace the letters onto contact paper, cut it out using an exacto knife or scissors (more on which below), and stick it on the glass. Then, dab the etching cream onto the glass, keeping in mind that any glass that is NOT protected with contact paper is going to come out opaque.
Sounds easy, right? It truly is. My tip about negative space vs. letter etching is this: for more complicated or multiple letters, it might be easier to use negative space, so you can use scissors to cut out the contact paper, which I find have more control than my exacto. The K above had to be cut out very carefully with an exacto, which takes much more time and is easier to mess up, in my opinion.