Dunno if it'll help but these were the steps that helped my husband do it for the first time:

1. Use a big screen, laptop or monitor, small screens like phones make it more difficult
2. First look at the middle point between both images
3. Now relax your focus to look "through" the screen at a point further in the distance. The two images will go blurry
4. If you're feeling the urge to re-focus on the images, hold it here for a second until you get used to the relaxed focus
5. As your eyes relax more and more, they'll naturally start to cross a bit and you'll see a third "middle" image start to appear. Don't do anything, just keep relaxing that focus and let it happen.
6. At a certain point you're able to sort of snap your focus onto that "middle" image, but it could take a few attempts to correctly judge when to do it without losing all your progress. It's a muscle memory thing, so practise makes perfect.

That may or may not make sense but it's the best way I know to describe it! Good luck.