My hero Bill Beatty shows how to do this on Vimeo.
http://www.metacafe.com/watch/1004040/melt_a_frickn_beer_bottle/ The process is relatively straightforward. Remove the glass plate and rotator from your microwave. Place an empty label-less bottle toward the back of a microwave. Warm the side facing you with a propane or MAPP gas torch, then hone in on an individual spot until it glows red. Close the door and start the microwave.