How to scale in AR as shown in the Unite AR demo using 'Camera Tricks'?

I've watched the Unite video and read the article https://blogs.unity3d.com/2017/11/16/dealing-with-scale-in-ar/

From what I've seen I'm still not even close to being sure about how this is done. I've looked in the forums and there don't seem to be any solutions to what I'm trying to do. I'm doing a physics simulation so its really important to use this scaling method.

The process would go:
Open app > Find plane > Place an a level prefab on the plane > Scale as necessary

I know there are forums but I haven't been able to get anything to work from them, just hoping to see if I can get any tips on how to make this works as I can't seem to make the 'camera trick' work at all.