You’re on the right track using Polaroid. The above suggestions are valid and useful, but using Polaroid instant is a classic method for achieving multiple exposure in the true sense, without having to wait for processing, or emulating fake multi-exposure with photoshop.
There are two different ways to accomplish this. It works with film, digital, and Polaroid.
Natural Light… If your ambient exposure is say ⅓0 sec @ f/5.6 for a single capture, then a double exposure would be half of that being 1/60 sec @ f/5.6 or ⅓0 sec @ f/8. This technique will cause your setting to be properly exposed, but your double exposure object to be half exposed, seemingly transparent by 50% against the ambient background. Get it?
Flash… Use a high shutter speed to black out ambient light and create a dark background. Use your flash guide number to calculate subject exposure for each individual frame. Add as many as you like against the black background. There is no compensation necessary in this scenario. If your flash says 15 feet @ f/4, then shoot every multi-exposure at 15 feet @ f/4, just like the chart suggests, and everything will be perfectly exposed.
Have fun with it. Using Polaroid is a great way to accomplish this task.