Step 1: Drawing the design - or finding one
The first step to creating your design is finding the pattern or image, or creating one yourself. When I was looking, a quick Google images search for vector-based grape vine graphics turned up some pretty good designs, but none that I liked as-is, so I wound up heavily editing one of the existing designs to get my pattern. While you don't need the vector graphic, it does make scaling work pretty well. After you pick out the image you are going to use, make sure you make it "laser-friendly." To do this, convert the image to black/white, not just grayscale. If you haven't done this before, in Adobe Illustrator, you can change the color scheme to grayscale, and then select the whole background region using the select by color tool, and clicking "Inverse Selection" under the "Selection" menu. This will select everything to be etched, which you will then want to paint black. Make sure you realize that the color will actually be reversed - etched glass is white, and red wines/dark beers/most other drinks are not, so etching something that is normally black and white will require you to switch your colors if you want it to look right.
On the rotary attachment on the laser, the laser will spin the piece as it goes, but it will treat the piece as if it is flat and starting from the top left, from the perspective of the image design. Therefore, the top of your glass is on the left side of the layout, and the top of your layout is the starting point of where the laser will start rotating around the glass. On the Crate & Barrel stemless wine glasses, the circumference is 8.64", and therefore the height of your design is 8.64 inches. If you are using a different glass, remember that you will need to measure the distance across the top of the glass, not the average, as the rotary attachment spins the top of the glass. Also, keep this in mind when designing your graphic - on a beer glass, you may want to stretch out the right side of each object (which will actually be the bottom) slightly to account for it being compressed on the glass.
One final note on graphic design for the rotary attachment, regardless of what you are trying to etch: if there's a taper, do not put a line around the glass, or anything else that must line up. Text wrapping all the way around is also a bad idea, if in the same font/on the same level. Due to a combination of possible dust on the drive wheels of the rotary attachment, uneven friction from the rear support wheels, imperfections in the glass, and a good old dose of Murphy's law, you will never get a line to actually line up. On my original design, I used a 7px line on the top and bottom of the glass, and it looked terrible. With the design I settled on (the same, but without the line) it was probably still off, but you can't tell. 1/16" is only obvious when your design points it out, but when it does...