Fold a bottom corner to meet the center of the opposite edge.
Take one of the lower edges, and fold it across the model to the opposite side, halfway between the other corners. It should create a horizontal line on the top edge.
Do the same process with the other lower corner, meeting up the top edges of this and the last fold. The corner should meet the corner that was created in the last step. Notice that there is a little pocket created in this fold, along the top edge.