The whole build from start to finish has been documented with both video and pictures. Enjoy!
Step 1: Subframe
A square will make your life MUCH easier when building big boxes like this. Cheap adjustable squares are usually just that, cheap and not square. A set of machinist squares are the best but the next best thing at a fraction of the cost are 1 piece framing squares. Even the cheap ones seem to hold a good tolerance thru their length.