Bring the 3D Stereogram image really close to your face (until you touch it with your nose).
At this distance your eyes cannot focus on the stereogram and they look somewhere behind it.
Now, slowly push the stereogram away from you, while trying to keep the eyes off focus.
At some point you will see the hidden image.
Another method is to take an object and put it behind the stereogram (about half of meter behind it).
Now, focus on that object while keeping the eyes looking at the stereogram.
Do you remember how is the feeling when you are drunk?
Your eyes don't look at the object arround you, but rather through them.
Well, this is actualy what we are trying to achive (look through the object, not get drunk, loll).
If you can manage to get your eyes off focus, at some point you will be able to see the hidden image.
If you're not train to look at stereograms, this might take you some time.
First time I've looked at a stereogram it took me like an hour
to finally be able to focus the eyes correctly to see the 3D image.