I found out how to do this.
First I have to put a layer into a group. Then, I right click on the Canvas attribute > Export Value and then give it a new name in order to create a new canvas which is automatically nested under the main canvas.
Would be good if this was clearer on the wiki!
I’ve observed a few possible inconsistencies.
If I export a group to a new canvas and then put a new layer in that group then the layer won’t always appear in the new canvas.
If I’m in the parent canvas and have the exported group selected then synfig won’t let me create a new layer. But, if I create the layer when the exported layer isn’t selected then I can drag it into that group.
If I create a new layer whilst in the new sub-canvas, go to the main/parent canvas and move that layer out of the exported group the layer still sometimes visually appears in the sub-canvas.
I can animate a layer both in the main canvas and in the sub-canvas but the way points don’t how up in both canvases.
This is quite a lot so I might try and make a video showing this.