The largest balloon was stuck straight down onto the card base, then I layered the next one slightly raised, by adding a couple of squares of card to the back before sticking it down. Then for the final one I added a square of card to the back, then a couple of pop-dots. The reason I stuck the card to the back was that I die cut them from foil paper, so I wanted to make sure they didn't get flattened.
And that was literally all there was to it. Really fast cards, which is always a bonus.