Multiple exposure photography is basically making layers of photos, whether they're the same or they're different or they're the same but at different shutter speeds, it's about stacking multiple photos to make one photo.
Step 1: If you have Picasa 3, open it up, if you don't have Picasa 3 you can download it here.
Step 2: Select the photos you want to make into a multiple exposure photograph, here are my examples (I changed the colors and they're at different shutter speeds but all at the same level, which is important otherwise if you aim to use the same subject, it'll turn out blurry):
Step 3: After you have selected the photos you want to use in Picasa, Go to the menu bar and click the one that says CREATE and within create select PICTURE COLLAGE. Within picture collage, select SETTINGS --> MULTIPLE EXPOSURE and it will stack the photos and make one for you. Select what size you want your multiple exposure photo to turn out as and select CREATE COLLAGE and in a short time you will have a multiple exposure photograph, here's what the 3 above look like when made into one photo:
And that's how you make a multiple exposure photograph using the same photo but at different shutter speeds and changing the colors of each photograph (which was also done in Picasa). Feel free to experiment and use different photos to make your own multiple exposure photograph, I intend to do that soon myself :)
For more examples click here