Initially, Sparkle will attempt to scale your elements to fit the narrower page format.
What’s required next from you is to rearrange the elements and scale them appropriately. As an example, you would probably want to scale the photo of the two models up and center the image within the narrow 320 px page width, and then move the image of the fabric color samples below the photo of the two models and scale that image up to fit the width of the 320 px page width.
Similarly with the text blocks, you will probably want to resize each block to fit the width of the 320 px page and stack them. You can then also redefine the paragraph styles for the 320 px page. (e.g., if a paragraph style on the 960 page called “Heading” is defined to be 18 point Helvetica Bold, then on the 320 page that same paragraph style can be redefined specifically for the 320 page to be 14 point Helvetica Bold.
A layout that works for a wide, multi-column layout will have to be reworked for a narrow one-column layout. Sparkle preserves all of your elements from device layout to device layout, but you must reimagine the layouts for the narrower pages and move and scale the elements appropriately.