I used my Cricut to cut the couple's monogram out of some leftover vinyl I had on hand. Cricut also sells vinyl cut to size for their cutting machines, but it is rather pricey. Alternatively, you could use letter stencils to trace the monogram onto contact paper (or just freehand them if you are feeling especially artistic). It is very important to make sure your glass objects are clean before you apply your letters/cutouts! I decided to have just the letters frosted, so I applied the vinyl surrounding the cutout to my glass, leaving the actual letter out. You could also have the entire surface etched with just the letters left clear, in which case you would apply the actual individual letters to the glass and spread the etching cream around them, covering the entire surface. Make sure to smoothe out all of the edges so no etching cream gets under your stencil (any glass that comes in contact with the cream will be etched). Etching cream is typically comprised of flouride compounds that alter the glass after contact (according to Wikipedia). Pay attention to the manufacturer warnings on the cream you buy because it can irritate skin, eyes, etc.
I used a small paintbrush to apply the etching cream to the letter "spaces," making sure to apply it rather thickly, filling in each letter completely. The particular brand of cream I bought needed to sit for a full five minutes before being rinsed off. I also cleaned the glass again with a streak-free glass cleaner once it was rinsed. When you do rinse the cream off, it will look like there is no etching. Don't worry! Once the glass is dry the etching will be more apparent.