How do you make a glass xylophone?

You'll need a glass cutter: http://en.wikipedia.org/wiki/Glass_cutter

Cut one sample bar out of the glass, typically, the one you'd like to be the lowest note or beginning of an octave, if you want an untuned instrument.

Accurately measure the width and length of this bar. When you cut your other bars, make them the same width as your first sample bar. To determine what the lengths of your other bars will be, use this xylophone bar length calculator: http://www.windworld.com/tools/freebar/index.html . You'll need input how many tone you'll want per octave. Typically this will be 12 ( http://en.wikipedia.org/wiki/Chromatic_scale ) if you also want the accidental notes ( http://en.wikipedia.org/wiki/Accidental_(music) ).

After bars are cut, you'll need to place them onto a body that allows good resonance of the bars, to maximize volume. Thin ropes are a good platform to lay them on. Make sure the bars won't slip and fall off, and you should be relatively ready to play. Use mallets that won't break the bars.