You take two pictures of the same subject. For the second picture, you shift the camera a little to the right (2-3 inches, or about the distance between your eyes).
The two images are then mounted side-by-side next to each other. Using a special viewer, or uncrossing your eyes (what's called "free viewing") you can focus one eye on each image and your brain merges them as a 3D image.
This simple instructable is to create a cardboard viewer, to make viewing easier.
If you just want to view stereo pictures created by others (or by yourself using a computer), you can create this type of viewer with any smart phone, iPod touch, or other device which has a large screen. The iPhone is particularly good b/c of the screen size and resolution (also b/c it's a camera).
Here is the home page for the iPhone / iPod app:
And here is direct link to download on iTunes:
Steremaker - iPhone app on App Store
Step 1: Make the cardboard holder
Cut out the holder roughly as shown using scissors or better yer an x-acto blade.
The slot for the iPhone should be a little longer than the iPhone is wide, and just thick enough to grip the phone when inserted.
I added a strip of ribbed tape used in bookbinding to the "far" end of the slot (the side that touches the back of the phone) to give it a little extra friction.
The important dimension is the distance from the front of the slot to the end of the holder - this sets how far from your eyes the image will be.