Make your stencil and cut it out. I used painter's tape, but I'd recommend contact paper.
For larger designs, you can put the stencil straight on the glass. For smaller details, I'd recommend cutting those out on the glass itself.
Slop on a nice thick layer of etching paste and let it sit. The longer it site, the clearer your image will be. I left it on for 2 minutes. Rise with water and remove your stencil.
Et Voila! (sans flash, here) Happy wedding!