Hold both sides/ ends, twist in opposite direction. It wont break, so use some force. You should see a figure '8' when its about to close. This means that you are on the right track.
I had the same problem, and the youtube videos plus the smaller version of folding seems not to work. Try this - fold from bottom left corner inwards and pull up - do the reverse way until the figure of 8 appears, and you should be able to keep it.