You have to do something like this 

But do it while looking at the 2 images. The 2 shots will become 3 and if you cross eyes more, they may become 4 even. Keep your crossed eyes angle where u see 3 shots.
Concentrate and keep looking on the middle one. It will become focused after some time and then u can see a 3D image 
Its difficult for some people to keep the eyes crossed for long time so it may not be possible for everyone to find the focused image as it takes a few seconds.