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.
Step 2: Stick the Phone in the Holder..
Slide the holder over the middle of the screen.
Now, when you look at the side-by-side image pairs from the end of the holder, the left image is blocked from the right eye and vice-versa.
Also, the length of the holder helps your eyes "un-cross" so you don't need lenses or other means to focus each eye on the different images.
You can try changing the length of the holder to make it more comfortable for you. Also you could try using better materials, add a handle, etc..
Try using in the dark, which helps un-orient your eyes to your surroundings and also makes the images much more vivid.
For general resources on stereo photography, search online - there are tons.