When clicking New Design in the upper right corner of the Feeds section, you'll be prompted to select from a pre-made template or open a blank canvas.
Clicking View all templates will direct you to a page in which you can see starter designs that preview with your source feed. On the left of this page, you'll see the template options you can choose from.
Clicking on one of these templates will show a preview of the template with 9 randomly selected products from your source feed to preview with.
You can also shuffle through different product previews by clicking the shuffle icon above the 9 preview images.
Once you find a template design you like, click Start with this template in the top right corner. This will copy the design into your account and open up the canvas to make any needed adjustments.