Double exposure is a classic poor mans photography effect. It is simply the process of using your manual SLR camera to combine 2 shots into one picture to create an overlapped dream-like effect.
Take your first picture in the usual manner.
Then press the film rewind button, often on the base of the camera
Slowly turn the film rewind knob until you hear a click, usually one full 360 degree turn clockwise.
Stop. The click means that you have rewound the film one full frame.
Next, advance the film forward twice. This repositions the first picture you took back into the exposure seat so that the picture you are about to take will appear overlapped onto your original shot.
Take your next picture as you normally would.
Congratulations you have now overlapped 2 images to create a double exposure. To continue experimenting with this effect start again at step one. To return to regular exposure simply advance the film ahead once.
There is a whole universe of SLR cameras. Be sure to check with your user's manual to find the correct locations of the knobs and buttons needed. For the more dramatic results try using bright or light colors in your first picture, and darker colors in your second picture. The contrast between the two should come out clearly.
Be careful not to accidentally open your camera while using the film rewind knob. On most cameras pulling up on the rewind knob will open the camera and expose your film, thus making the film unusable.