Just pick a singlefold card, cover the inside right-hand panel with paper (I used plain lime green but you could use patterned paper).
Die-cut a hole or two in whatever shape takes your fancy . (At risk of stating the obvious, I chose two sizes of circle).
Now choose dies, that just fit over the edges of the cut apertures, so that there is enough to glue to the front of the card, but so that most of the die is over the hole, Cut them out, fix in space.
The finished card has a lovely 3D effect when it is opened or displayed in this way. I used butterflies but you could equally well use tools (hammer, saw etc) or flowers, or words or anything really, as long as they fit with the holes that you cut. As a result, you could make a card like this for any occasion!