What works for me is what I used to do in the 90s with those magic eye posters and that would be put my phone up very close to my nose and slowly draw it away as I focus on the dead center of the screen. Or in this case the center of the two pictures and your eyes will naturally merge the two into a clear view of the 3D image. Basically, if you do it right, your brain will trick itself into thinking your eyes aren't looking at something correctly and will ignore the exterior and combine the two images in the center. It's my theory that your brain thinks it's supposed to see something 3 dimensional (as one does) and will compensate for the fact that the user of the brain is being stupid. (I don't mean you) When I just try to cross my eyes and never works. But if I pull the phone very close to my face and slowly pull it back it always works. That's not a scientific fact. That's just a uneducated guess. Lol but hey it works for me.

Edit - I back this theory up for myself because if you finally get it in 3D and you try to zoom in on it, like zooming in on a phone, your brain will do all kinds of fucking wonky shit because the rest of the pictures aren't there and all of a sudden it doesn't make sense. You would think you could zoom in on it put your brain. Actually knows how to think you don't lol