Still rough today. Don't know if it's the ME or I have a bit of a bug - the old goat was rough for a couple of weeks, and princelet has the sniffles.
So here is a Christmas card I made earlier. Yes I know, sticky back plastic - too much Blue Peter as a kid, and all that.
Can you see the shape? It's an over lapping gate-fold, cut to a window arch shape - but the flaps are also cut to window arches so it folds out to a triptych.
Experiment with paper until you get a good shape - then make a card template and there you go!!