This can be very useful if you have a large mockup image of what your finished Experience should look like.
First, you will want to add that template/mockup image to your Creator Studio account as you would any other piece of imagery.
From there, open your Experience and switch to the Background layer via the Layer Switcher (to the left of Rulers on/off switch, just above the canvas).
While in the Background layer, add that image to the canvas, then switch back to the main Scene layer of your Experience.
Now inside the Scene layer, build your Experience as you normally would and layer your Widgets on top of your template/mockup image. Once you are done building your Experience out, be sure to just switch back to the Background layer and delete that image there.