This is a great bar trick and one worth doing well.
The first part is the trickiest: you must break the shot glass down into its component parts.
You can do this one of two ways, the first being the coolest:
Now comes the fun part: start eating
Glass is made of sand, and the more you chew the glass, the closer to sand it becomes. The important part is to keep the glass away from your tongue, cheeks, gums, etc., until it has worn down enough to become safe. The watchword is chew, chew, chew.
When the chewing part is over, all that's left is to swallow. It's as easy as falling off a flagpole.
So, there you have it. A great way to impress the ladies, win a few wagers and maybe even get a free drink or two. You can thank me later.