Instructions
Place a fretting finger into the glass slide. Press your thumb on the same hand against the back of the guitar neck.
Press the glass slide over a fret. Your don't need to push down hard. Pick or strum one or more strings with the other hand.
Touch the string behind the slide to dampen the notes. This will give you a more solid sound.
Place your free fingers on the fretting hand between frets to create notes. This isn't necessary, but it can help you create a different sound.