It is very simple, but also very fiddly, to do a double exposure with any camera, and it can be done in a few easy steps.
Firstly, take a picture as normal, but with the iso set to double the number of the film speed (e.g for 400 speed film, set the iso to 800) this is so that the image isn’t overexposed, as there will be double the amount of light hitting the film than there normally is. This will be the first image of your double exposure.
Now comes the fiddly part. First, wind the rewind lever on until you feel a bit of tension, then stop. Then, hold one finger down on the rewind button on the bottom of your camera, and hold another finger behind the rewind lever so that it doesn’t move. While still holding those two things, advance the film with whatever is the easiest, maybe another finger if you feel flexible enough, you could get a friend to do it, but my personal favorite method is to use my teeth :)
This will have cocked the shutter, without advancing the film, enabling you to do some awesome double exposures!
Here are the results i got from doing this on my AE-1:
Although, this method is not fail proof, occasionally the rewind lever will slip, leaving you with misaligned frames like this: