I find parallel viewing difficult but cross-eyed quite easy. If you can raise a finger and squint and consistently manage to see two fingers, you should manage:
Do it again with two fingers in front of you, you should be able to see four fingers. Adapt how much you squint in order to see "three" fingers (i.e. two of them overlap). (Practice to perfectly control how much you squint: you should start at zero squinting and increase slightly until you achieve the overlapping.)
Once you can do that easily, replace your fingers by the SIRDS generated on that website. Instead of two fingers, you use two features on that image that you see are repeating. You choose two of them that are next to each other. You do exactly as with your finger: turn those two features in "three". Now I think this is the harder part: as you do that, you have to relax and let your view adapat so as the image becomes perfectly neat (not blurry at all).
Do it again with two fingers in front of you, you should be able to see four fingers. Adapt how much you squint in order to see "three" fingers (i.e. two of them overlap). (Practice to perfectly control how much you squint: you should start at zero squinting and increase slightly until you achieve the overlapping.)
Once you can do that easily, replace your fingers by the SIRDS generated on that website. Instead of two fingers, you use two features on that image that you see are repeating. You choose two of them that are next to each other. You do exactly as with your finger: turn those two features in "three". Now I think this is the harder part: as you do that, you have to relax and let your view adapat so as the image becomes perfectly neat (not blurry at all).