The card on the left is super easy and super fast. I just used markers and stamped a sentiment over. It is on shimmery white paper so it has a nice, subtle sparkle.
The card on the right is very simple as well, using exactly the same products but I decided to decorate the sides with faux stitching. I was using a piercing tool and an old Stampin' Up!® pierce guide but if I didn't have that I'd just use a ruler. And if I was in a hurry and didn't want to make the stitches, then just a simple line made with the thick side of the marker would look great too.
I put together a little clip showing you how I made both of them. Both cards are standard size. The bases are half A4 folded in half and the shimmery white pieces of cardstock are cut to create a 2mm base border and measure 10.1 x 14.4cm).
I used these products to make the rainbow duo cards: (Click on any of the products to shop online)