Please check out this document: responsive.zip (1.6 MB)
Three difference layouts satisfy three screen breakpoints:
320 to 400px
400 to 1200px
and finally, 1200 and up.
I also set Flexible Layout properties on the scene by checking the width / scale box in the document inspector:
And on some elements I set width scaling so they reach the edges of the document:
There many ways to do this, but the one thing that will likely not work well without some additional work is scaling on the height dimension.
If your document scales up and down for mobile devices and desktop screens with the same layout, your text will likely not look right in the size you don't design for --it will either be far too large or far too small.
It would be great to know what you're building and what type of content you are working with if this solution doesn't work well for you. I'm currently working on a video tutorial for this and I would like to find a way to create a tutorial that helps most people working on most types of documents if I can.