I cut the raised portion of the bubble off the card using a sharp craft knife.
You can't really see it in these smaller pictures but it leaves the flat part of the bubble glued to the card.
You can see it on some of the cards if you look at the larger pictures on Flickr:
https://www.flickr.com/photos/cheese...10883/sizes/o/
https://www.flickr.com/photos/cheese...71096/sizes/o/