How do I enable print frames?

[Walrus]

I added a border to the print_wrap element via CSS and it works fine but in responsive mode it's offset and looks like a mess.

There is frames feature according to documentation but I cannot find anything in Settings, or online documentation, on how to use it, other than a sentence on the Print previews page in Settings, which I cannot fathom.

As I don't want to sell framed prints, a simple border is all I'm after but how do I enable it so it works in both desktop and responsive modes?

TIA,
Walrus.