Well, i have seen these "bag a lope" so many times but neve tried to make one until today.
You start with an envelope, i used one that measured 16cm x 23cm. Stick down the flap to start.
Then your score both sides and the bottom at 2.5 cm in from the edge then cut it down to 16.5cm x 16.
Fold the scored lines over and stamp your envelope on the side that the folds are underneath.
Then turn over and turn the scored lines the opposite way and stamp this side also.
Open the envelope out and at each side and press on the bottom so that it goes flat and looks like this