There is guide on the Wiki under Usage Tutorials. Good luck.

Make them different temperatures.

I can't make link 'cause I'm my phone:
https://powdertoy.co.uk/Wiki.html?id=using_wifi_element

You need to heat them at approximatively the same temperature. It's simple, even me I did it.

@mat212
And once you have heated them to match each other, connect the sending end to PSCN and the receiving end to NSCN.