The easiest fun fold I know of is what I call a 'Z fold' card. You make a standard card and add one extra score to the card front. I am in the US so all my card are A2 cards--sorry to my Stampin' friends in other countries.
Starting with an 8-1/2" x 11" piece of card stock you have 2 choices:
1- Cut in half to 8-1/2" X 5-1/2".
2- Score on 8-1/2" side in half at 4-1/4" and half again at 2-1/8"