Hold the image right up to your nose, relax your eye muscles and slowly move the image away from you. Keep your hands steady and don't let anyone interrupt you. It may even be an idea to find a quiet room. Without focusing your eyes you will soon see the 3D image. Some images may appear sooner than others depending on how far you hold the image away from you.
Many people use this technique first then progress to other methods as their observing abilities improve.