To make this work, I need to make partial cuts along various lines. It will be much easier for me to show you than try to describe them so I have highlighted the cuts in red below.
Once these cuts are made I pre-folded and creased all score lines, EXCEPT line 2 & 5 on the long side, by folding over, pressing and then folding back.
Easy enough so far. The next step is a little more complicated so I have provided diagrams and included a real time section in the video so that you can follow along if you need to.