I recommend watching stereograms on a desktop monitor, a laptop or a tablet. It is rather difficult to focus on a mobile phone screen.
Get closer to the screen (20-40 cm distance off the screen, depending on the screen size), looking at the image normally. Then slowly start to squint. The image will start to split in two. Slowly squint harder and at some point the two images will join in one. At that moment you will see that the notes are closer to you then the background.
If this explanation doesn't work for you, here is a web site with stereograms. The explanation on how to watch them is at the bottom of the page.