The first thing to do, once you have chosen your glass, is to clean it. Obvious I know but it needs to be free of grease and fingerprints. Alcohol does a good job. Not drinking it, rubbing it on the glass, that is.
If you have tried lettering around an obviously truncated shape before, you will realise the problem. Lettering around a tube is easy but it is complicated when there is a narrowing at the base. The letters are flat, the object is not. Think about a straight shift dress and what happens when you belt it. You get gathers. Not desirable in this case.
So, you need a template of a cone. There is an excellent site with free templates (for cones and boxes and the like). It will save to PDF or SVG - firstly, here is how it is done in PDF for those who don't have designer edition.
Download your pattern from Truncated Cones
It is a simple measuring job to get the size you need. A word of caution, although it will import as a PDF, it imports in bits, so group it! Duplicate it and put one copy aside. Draw an ellipse that matches the curve in the centre, it's quite easy to do. Duplicate that and keep to one side. This is just a precaution but a good one!
IF you can import as SVG, It is a bit quicker, you can separate the lines to duplicate the centre dashed line, so don't have to bother with the ellipse.
Cut out the template in paper or card and the letters in vinyl.
Weed the letters and use masking or transfer tape to transfer them to your glass. Cut the whole paper template and insert into the glass as a guide to follow for placement. If you use a sketch pen to draw it before cutting, you won't need to do a print and cut. Use a temporary marker to mark the horizontal and vertical on both the template and the vinyl to match them up. Remember the base of the glass will put the depth out slightly if your template is inside.
Don't just stick to letters, other designs can be fun, especially for one off occasions. Spiders for halloween, hearts for valentines.....
If you want something more permanent, check with your vinyl supplier. Some have even been known to go through the dishwasher regularly!