Step 1: Sit about 2 feet away from the computer screen, facing it directly. Place your index finger about 6 inches in front of your nose, and close your right eye. Now, line up your finger such that it appears, from your left eye, to be directly underneath a particularly prominent scene element (i.e.: a person's head).
Step 2: Next, without moving your finger, open your right eye and close your left eye. Your finger should be lined up such that, from your right eye, it is directly underneath the bottom point of the other field's representation of that same scene element.
Step 3: Now open one eye at a time, back and forth. You may need to move your finger around slightly forwards or backwards or laterally until it simultaneously points to the left-eye and right-eye representations of the same scene element in each respective field.
Step 4: Once you have that, open both eyes and look at your finger (which hopefully hasn't moved). Just above your finger, very much out of focus, should be an out-of-focus image of the scene.
Step 5: Now comes the tricky part: Keeping your eyes aimed at your finger, or just above it, try to relax your focus on the scene right behind your finger. If you can do this (it is easier for some people than others) then you should be able to see the image scene directly behind your finger stereoscopically.