I also worked on a tunnel card for a Fiskars challenge. This is a new type of card for me so I started by using Fiskar's round template to make a circular card. Then cut a hole out so it has several layers with holes so that it tunnels. The tricky bit is making an interactive card by placing flowers with a tab that moves things like fairies into the tunnel. Here are some pictures of a template I made. Didn't want to waste good card stock while I'm still designing so it is all white.
A rough tree made with card scraps.
A faerie is dropped in by moving a flower in outside of card.