Pull the flap open a bit and pull on paper where marked by circle.
Paper is pulled out- note that the right side will not lay flush until the next step is completed, this is not shown in diagram.
Now you are reversing the folds from the previous pic. The one in the middle that was a mountain becomes a valley and the outer valley becomes a mountain.
A look at the inside once the folds are reversed and the flap is being closed back up.
Done! Now fold point to the left and repeat on opposite side.