Dies are from the latest issue of Die-cutting magazine
Sentiment stamp is by Woodware
Card folds (fairly) flat and is then 17 cm x 17 cm so fits in a 7 inch envelope.
A4 cut to 29cm and then score at 6cm and 9cm, turn 180 degrees and score again at 6 and 9cm
Fold side panels in and then cut card to 17cm height, the strip you have just cut off is then flipped over and adhered (the 'cheers' bit)