Today's tip really is a quick one: When creating a shape card, cut an extra full image for the front. That way, you can glue the image to the front of your folded card and will not have a flat line visible at the fold.
You can see the folded line at the top of the black base of the card. I cut an extra image (in brown) to glue on top in order to have the entire football without a flat portion.
And here is the completed card: a shape card with no flat-edge visible.