Instructables

Glass guitar slide using a beer bottle and a rudimentary tool.

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Step 2: Start filing.

Remember, you are working with glass, which can be sharp! Don't cut yourself, Don't be reckless and do this all gloveless like pictured, he almost cut his finger on glass (But in his defense he didn't think it would work...)
Wear a mask too, will protect you from inhaling all those dangerous glass dust bits. :0D
Eye wear would be nice as well, No one wants glass in the eye.

Start off just filing away a bit at a time, gradually turning the bottle as you do so, keep going over the marks you are making, occasionally tap it on the edge of a hard surface, carefully so you don't shatter the glass, Keep this up for about 15 minutes or so, depending upon how much effort you are putting forth and it should just pop off the bottle.

Step 4: Extra step

If you want to go for a slightly more finished look, you can go a step further and file off the lip of the bottle, Mind you the glass is much thicker here so it will take much longer unless you are all fancy and have a dremel tool or a glass cutter, but if you have those, then why would you click on an instructable showing how to make one with simple tools?