Oftentimes we have to work on odd-shaped glassware that presents its own can of worms when it comes to registration. Not only is the item to be etched not flat, but in most cases, it also does not have straight lines.
If there are only names and a date to blast, we can usually find a spot on the glass just straight enough to apply the stencil without major problems. In those cases, mark the glass for the placement of the stencil. This can be accomplished by finding the right items that let you put your marker onto them to be able to mark the glass in the right place, by rotating it in front of the marker. We used to do this frequently until a gadget was made that allowed us to adjust the marker in various positions without having to search for objects to do the same.
The tool makes the marking job easier and faster. Once the marks are made, you can apply the stencil onto the glass.
Note: this item is not for sale, but the example serves to inspire ideas to build your own tools.