I'm not sure why I have the diagonal edges when in the video it goes down as a rectangle. Please help, can't get it to work.
It looks like you moved the selected face down with "g". In that case surrounding geomety stays connected and will get angled like in your screenshot.
Instead of moving you need to press "E" to extrude the face downwards. That creates new geometry and you will get the rectangular shape.
You're right... I missed it in the video. I was just trying to move it... Thanks!