Last week Eleanor and I made invitations for her birthday party. I got the idea from a card I received ages ago; the original just had one large butterfly, but I modified it as my butterfly stamp is fairly small.
The first stamp needs to be carefully positioned; half above and half below where the fold will be. We used a rainbow stamp pad.
Then you just cut around the upper half of the butterfly with a craft knife, lay it face-down and carefully fold the back over (leaving the butterfly sticking out, unfolded) and press in half.
I used lightweight card and ran them through the printer with all the party details first.