This method works for me. I hope it’ll help you too!
Relax Your Vision
1. Put stereogram in front of you, let’s say 30cm(1 foot) and start relaxing your vision. After a bit of time your eyes will feel little strained and you won’t be able to concentrate anymore and the picture will start to become blurry and doubled.
2. This will make your eyes to diverge on different angles. At some angle you will see that a hidden image appears. When it happens eyes should stick to this angle and you’ll be able to see hidden 3D.
3. Once you can see blurry 3D picture, keep looking at it this way until it becomes sharp and recognizable.